hows the familiar meant to work

ive notice that the writers of 5e (mearls, crawford, perkins, et al) are reluctant to tell you what to do with familiars and like animal companions and stuff in interviews - like i think that the strictest reading of the rulebooks wants it to be

familiars (only as a class feature, not useful in combat)
animal companions (only as a class feature, useful in combat)
and now sidekicks (anyone can have, useful in combat)

and like, as written the rules sort of deliver these things, sort of not

and in interviews they'll be like, of course, anyone can have a familiar, of course - and i can see in streams and adventures that this is the intended way to play - and now because theres an explicit system in sidekicks, anyone can have a guy whos useful - and in a game that doesn't use sidekicks, we can see the same system is still intended in late-release beastmaster class features like the ua beastmaster ranger and the artificer

and the boring answer is like, they want you, the dm, to decide, because of course i can decide stuff, i can make up whatever dumb rules i want, but hows it actually sposed to work