There’s no need to register an account with Ubuntu at all. You have no idea what you’re talking about. You don’t need a pro license to get updates for an LTS for 5 years of support. The “base packages” are both the “main” and “restricted” repositories - it isn’t just a few “core libraries” as you seem to think.
Really? So why does apt tell me that I need <some blabla that usually means “give us your money”, don’t remember exact wording> to get updates for more packages than it has downloaded each time I run apt update? I have latest LTS (22.04) on my laptop. Maybe you have no idea what you are talking about? I could get any updates until recent (year or two? I use that laptop only occasionally, so I don’t remember the exact time), but now it is clear that Canonical goes the same way as RedHat/IBM.
I would guess that it’s the same though - “main” repository is what they cover. Similar to Ubuntu.
You are wrong because Debian’s main is not similar to Ubuntu. Debian has no universe repo, all FOSS packages go to main.