It's very variable (disclaimer: I'm a SALT student, also a parent of a child with quite a complicated little mixture of communication issues) and yes, in the early years a lot of it IS common sense... but unfortunately to lots of families it's NOT. I had the "have you tried speaking to your child" conversations - introduced them to my other child who was a very advanced and early talker (and has never shut up since) and got an "ok, we'll skip that part of the appointment then!"
Background noise minimised because so much language is picked up by that incidental exposure and really focusing more on the back-and-forth conversation turns than doing the kind of performance parenting constant commentary that'll get you a mention on MN (and doesn't let the kid get a sound in edgeways)! At this age lots of it IS parent coaching and stating the blooming obvious really - and giving later talkers the chance to catch up.
And yes, as someone mentioned - there's a tendency to focus so much on the words coming OUT rather than the language that's going IN and being processed inside that little head (DD2 was very like that - couldn't articulate it, but blooming heck it was all going in).
The one thing I'd say is that it can be frustrating as fuck - you feel like you're getting nowhere and slogging away doing all the homework week in week out and it's only when you look back to where you've come from, rather than thinking about the ground you've still got to cover, that you realise the progress you're making - I've had to mentally slap myself around the head a lot of times to remember to do that with my own kiddo.
It's worth looking around if you really feel like you're getting nowhere though - and yes, I do hear the long waiting lists - I've been trying to get some updated reports done on my own child and it's taken me months to find a blooming SALT to do it - the irony of which does not cease to amuse and frustrate me. I know round here you're looking at September for private therapy start dates now - the NHS backlog from Covid all seems to be trying to go private and the whole house of cards is looking distinctly wobbly right now.
And @rachelvbwho - my little girl started school with about 25% intelligible speech (she has verbal dyspraxia), by age 5 there were loads of sounds still missing that she couldn't say (bear in mind that there are some that aren't really "missing" until well past that point) and I used to feel so down about her not being on a par with her peers - but she DID nail it, left infants speaking really clearly (it's always likely to have some quirks to it) - regressed a bit during bloody lockdown - but she can more than hold her own nowadays and make her views on anything and everything known!