I don't see how I can start an E2EE chat from my PC. If a feature only ever exists on phone it does not exist.

https://github.com/telegramdesktop/tdesktop/issues/871

...in secret chats, that are only available in 1-to-1 conversations via mobile apps.

So, not on by default (unlike Signal), no group chats (unlike Signal), no support at all in desktop apps (you've guessed it: unlike Signal).