You could use dpkg but I just use apt.
cd to the directory where the .deb file is, then just sudo apt install ./namofdebfile.deb
No. They both work differently for different use cases (snaps being more versatile), and so unless you’re running a server (where Flatpaks are useless), you should just use snaps, and if Flatpak becomes useful, you can enable it, add repositories, and run them.
But mainly you should just use Ubuntu Software (or Ubuntu App Center for Ubuntu 23.10 and onward) and do as much as you can, and not worry about Flatpaks unless you have something you really need that isn’t otherwise available.