Is there a simple explanation of the differences or use cases of the three versions mentioned?
I need a secure digital system whith python support, internet access which only shows a browser. It should run on a Raspberry Pi. There is no commercial interest.
Ubuntu's main releases use deb package (deb are also used by Debian). All deb based releases are year.month in format, eg. Ubuntu 20.04 LTS Desktop.
Ubuntu Core is not a deb based system thus uses a different format, namely year only, eg. Ubuntu Core 20. These releases can use snap packages only, no LTS gets mentioned normally as all releases have 10 years of support.
Minimal can mean two things, there is a minimal install option where the full install occurs (ie. installed image is written to the disk) then if you selected a minimal install, packages listed on a list are removed (creating the minimal install option).
There is also a netboot or mini ISO that is a network loader; it boots the system and asks what type of install & downloads all packages from the web & installs. See https://wiki.ubuntu.com/Minimal You may have meant that, but a minimal-install option is generally more common and what you mean.
The Ubuntu base is the same base found on all systems (be they desktop, server, core etc).
There for sure are many other things to consider
The best for you depends completely on your intended use, applications etc.. which is a very opinion geared question not encouraged here (better for forums, Ubuntu forums can be found at https://ubuntuforums.org/.
