Thanks for the video. The script makes it indeed super easy to install. 
For those who prefer to manually install the controller on Debian 12, I have written down the necessary steps below, which aren’t too complicated either. The only annoying thing is the dependency on libssl 1.1, which is due to the ancient version of MongoDB required by UniFi 
Prerequisites:
sudo apt update
sudo apt install -y gnupg curl wget ca-certificates apt-transport-https
Download and install libssl1.1:
wget http://ftp.debian.org/debian/pool/main/o/openssl/libssl1.1_1.1.1w-0+deb11u1_amd64.deb
sudo dpkg -i libssl1.1_1.1.1w-0+deb11u1_amd64.deb
Add the UniFi and MongoDB repositories:
echo 'deb [ arch=amd64,arm64 ] https://www.ui.com/downloads/unifi/debian stable ubiquiti' | sudo tee /etc/apt/sources.list.d/100-ubnt-unifi.list
sudo wget -O /etc/apt/trusted.gpg.d/unifi-repo.gpg https://dl.ui.com/unifi/unifi-repo.gpg
echo "deb [ signed-by=/usr/share/keyrings/mongodb-server-4.4.gpg ] http://repo.mongodb.org/apt/debian buster/mongodb-org/4.4 main" | sudo tee /etc/apt/sources.list.d/mongodb-org-4.4.list
curl -fsSL https://pgp.mongodb.com/server-4.4.asc | sudo gpg -o /usr/share/keyrings/mongodb-server-4.4.gpg --dearmor
Install UniFi: (MongoDB and the rest of the dependencies will be installed automatically)
sudo apt update
sudo apt install -y unifi
Sources:
https://packages.debian.org/bullseye/amd64/libssl1.1/download
https://www.mongodb.com/docs/v4.4/tutorial/install-mongodb-on-debian/