How come John Cena can just announced he'll be at Elimination Chamber and others need to face off for a spot?

I got back into wwe after nearly 20 years earlier this month when it came to Netflix and I have been really enjoying it since. While I’m entertained by the storylines, I’m a bit confused as to what the “rules” are.

John Cena had a press release the other day where he announced he will be at elimination chamber, but tonight on raw there were qualifier matches. Is this just because Cena is goated and can choose where and when he will compete? Or is it just the way it’s being written and there isn’t really any rhyme or reason to it?

I knows its entertainment and not a full fledged combat sport, but theres weird things like title reigns and matchups that I dont fully understand either lol. Like, if a wrestler is just really popular and WWE wants to feed off that popularity do the writers just decide XYZ wrestler will keep holding the title indefinitely, or is it the same as my above point and there isnt really any rhyme or reason in how decisions are made.

Thanks in advance and sorry if this is a dumb question!