For calendar, I just use Proton Calendar. For notes, it depends on what sort of notes. Quick ones for reminders, jotting down a thought, etc. I use PocketPlan, which is an AIO notes app, todo list, birthday reminder and shopping list app that's absolutely fantastic.

I’ve never found a better calendar app than aCalendar+. For notes I use Obsidian.

https://github.com/FossifyOrg/Calendar - on f-droid, simple and gets the job done https://github.com/OmGodse/Notally - on f-droid, simple with no bloat

I nave always user Obsidianfor my notes (wich are mainly for D&D and personal project, not anything work or school related) and I'm extremely happy with it. It works completely offline, you can make CSS snippets yourself if you know how to, it's very customisable and you could even avoid making an account for the cloud sync using other tools (I use synchthing). It doesn't use LLMs in any way, it doesn't has any ADs and it's also free

I've been using Etar for years. Can't complain. Edit to add that it's an open source calendar app available on F-Droid.

I'm using Markor for notes and it's ok though missing some stuff. I'd like it to support Org markup and git versioning.

I use the original Simple Calendar (now FOSS Calendar, I think), and Quillpad.

Markor + syncthing works for notes.

I moved away from calendars and use tasks.org with due dates for all my events. That's android and can sync to multiple devices.