So the next paragraph is my original answer, and everything after is all the notes I took. Everything check out? It’s overkill, but it includes the answers to questions (which I left out) that popped up in my trek to fully understand. Great answer by the way, thanks!
In the Turing Pi example, (default) bridged mode RJ45s (as defined in advance on the switch by Turing Pi developers) accept/deny frames not destined to connected CM4s (for example), while the switch is a normal, but hardwired, switch which transmits frames as appropriate, meaning the fact that the RJ45 jacks are in bridged mode is only relevant to them, not the CM4s. Tautological! It seems I was looking for (additional) meaning where there was none. Oddly, I looked up the model number of that chip and found nothing, even on RealTek’s own site, except a couple with suffixes N-VB-CG and MB-CG. Weird.
Layer 7 (Application) SMTP, FTP, Telnet
Layer 6 (Presentation): Format Data, Encryption
Layer 5 (Session): Start & Stop Sessions
Layer 4 (Transport): TCP, UDP, Port Numbers
Layer 3 (Network): IP Address, Routers
Layer 2 (Data Link): MAC Address, Switches
Layer 1 (Physical): Cable, Network Interface Cards, Hubs
Memory Mnemonic: All People Seem To Need Data Processing
(Mnemonic and basics from some of the following is from a “Certbros” YouTube video I found on the difference between hubs, switches, and routers, and other sources online reworded in a way that makes sense to me with lots of extra information as well.)
Modem (Layer 1): Establishes a network connection with an ISP. Early modems dialed a gateway which carried the signal to establish a connection between your home computer (which was assigned the globally accessible IP address) and the Internet, this meant your computer was accessible to all, unless protected by firewall software. A router could be used to sit between the modem and computer and helped provide some protection by segmenting the Internet from your home, providing you your own LAN, as well as allowing multiple devices to connect to eachother and the Internet at once. Most modern modems also contain routing hardware (at a minimum) to segment your home network from the Internet, and multiple ports forming a simple basis for your home LAN (though a LAN doesn’t require a Router if not connecting to another network or the Internet), allowing you to have your own LAN without additional hardware.
Hub (Layer 1): Physically connects devices together into one network, blindly sending traffic to all connected devices except the original sender, connected devices themselves are responsible for accepting or rejecting traffic (has several physical ports). Network hubs in hardware form were superseded by bridges.
Bridge (Layer 2, MAC Address only): Connects network segments (layers 1 & 2) together allowing for more devices to form one same network (e.g. one LAN, this is why both ports on the Turing Pi 2 can’t both be connected to the same network Switch or Router), a Bridge is also a Hub. A Bridge (filtering) can be configured to occur between NICs, ports, or both. When a frame destined to a never-before-seen MAC address is received by the bridge, it forwards it and, if no reply is received, drops future frames to that destination. When a reply is received, the MAC address is added to a MAC address table and immediately forwards future frames destined to the same MAC address. Network bridges in hardware form have been superceded by switches, though they are still used today in software (e.g. virtual machines, or, bridging physical ports in firmware which are connected through a switch chip which also connects other systems which are in their own network segment as in the Turing Pi).
Role & terminology disambiguation note: Bridges with only two ports are commonly referred to as such and not as a Switch because the only decision to make, using the MAC address table, is whether to forward data.
Switch (Layer 2, MAC Address, Port): All Switches are Bridges, but have (potentially many) more than two ports. This forwards a frame with an unrecognized MAC address (like a hub) to every port, adds the MAC address to the MAC address table (like a bridge) with the port which replies, and sends all future frames destined to the specified MAC address to only that physical port (sorting, to optimize transmission speed). This also means no devices on the network (at Layer 3) will be assigned an IP address if the switch’s upstream link is not directly or indirectly connected to a Router.
Switch (Layer 3): Contains all the same functionality as a Layer 2 Switch, with additional features such as dynamic or static routing, and VLAN awareness.
Switch (Layers 4+): Same as above, but capable of also doing switching based on the information available to it relative to the OSI layer the switch was designed to work on, as well as layers beneath it (see “Multilayer switch” on Wikipedia).
Managed vs Unmanaged Switches: All managed switches have IP addresses (for management purposes), Layer 3 and above switches are, by definition, managed, thus have an IP address and need to be directly or indirectly connected to a Router upstream to perform their Layer 3+ routing.
Router (Layer 3, IP Address): All Routers are (layer 2 at a minimum) Switches, and serve to establish connections between networks (e.g. different LANs). Most often has several ports, but may have as few as two (upstream, downstream) for Routers with a specialized purpose that goes beyond a basic Router package (e.g. Pi-Hole). These use MAC addresses when communicating within the same LAN (whether or not it’s the same network segment) and IP addresses between networks. Routers often have built-in DHCP servers (etc) to ease or automate configuration at device connection establishment time.
Random note which I’m less sure of:
Network design (spine-leaf vs three-tier) results in optimizations targeting different aspects of a network, based on use case.
Three-tier (traditional) is okay for simple networks, where devices need access to eachother more-or-less equally (if at all), with access to other networks (e.g. the Internet) provided for at the top.
Spine-leaf allows fewer hops for traffic between client and server (e.g. storage or compute to client, and servers to eachother) on the local network, with connections to external networks being provided for at the lowest possible point of impact (leaf switches), if servers shouldn’t be directly Internet-accessible for example), though this is at the cost of extra cabling and hardware.
Network segment vs network (LAN):
A network segment is a network of devices, all of which are connected through the same hub (layer 1). Multiple connected network segments are a LAN in and of themselves. Once a router (acting as a router, not a hub) is connected to a device in the network, this creates a break in said network, such that other devices connected to the router upstream are addressable and accessible, but not part of the same LAN. LAN devices are thus also accessible to the upstream network’s devices if no firewall is in place.
(Apologies for the disorganized notes.)