I read Ulysses last year and enjoyed it but this "companion" book was indispensable:

https://bookshop.org/p/books/ulysses-annotated-revised-and-e...

But also the free (online) Joyce Project with hyperlinked annotations:

http://m.joyceproject.com/chapters/telem.html

I wish I'd bought this version when It came out. It's pretty expensive now.

https://bookshop.org/p/books/the-cambridge-ulysses-the-1922-...

And I cannot recommend Frank Delaney's Re: Joyce podcast enough, sadly he passed away before completing the project

https://blog.frankdelaney.com/re-joyce/