^^ Joshua Bardwell's channel is a wealth of knowledge with regard to betaflight, he has a few iNav videos here or there too and just a ton of product review vids since he gets a lot of gear sent for free for the review/exposure.
I'd suggest as a first thing just buy a smaller quad off the shelf (something like a tiny hawk 2 or similar 1-3" prop sizes and smaller frames).
Spend some time deciding what transmitter to get first since it's the first big time cost and thing you'll likely stick with for the long term (Transmitters and goggles will last across tons of models of aircraft and whatnot so is good to think of as something of "investment" eventually they'll be garbage but should expect them to last for at least a year if not a few years while learning to fly and whatnot).
Regarding transmitters you basically need to grok that there are different radio protocols and your transmitter and receiver need to "bind"/pair so need to be using the same frequencies/protocols in order for that to happen (basically the transmitter/controller you get will dictate which quads it can bind with out of the box). I personally have and like the TX16S from radiomaster, main things to look out for, 1 does it have a radio module bay where you can swap out the transmitter radio, if not is the baked in radio the one you plan to stick with (I am a big fan of ELRS protocol/transmitters/receivers for control, but lots of stuff is FrSky or Spektrum or other brands/protocols used so is a good thing to start wrapping your head around a bit).
Second big purchase if you are going to be flying FPV will be goggles, is a similar kind of explanation, lots of companies competing, a handful of digital options that basically only work if you buy the video RX and TX from the same company, for the most part analog video is cross compatible in terms of brands and is not encoded so just like old school analog TV over the air is pretty open for anyone to build receivers or transmitters to work with.
