I saw again recently someone suggesting that a first time AR15 buyer build their own AR15 so that they would be better familiar with the parts and operation. I think a person could learn the parts and operation of a firearm just fine without building one.
By all means build a custom AR if you want too, but I highly recommend buying a good factory built AR15 for your first one. Having built quite a few ARs myself, and seen many more built, there are all sorts of mistakes a person can make. I’ve seen incorrectly aligned gas blocks, gas tubes, hammer springs reversed or under the trigger pin (allowing the trigger pin to walk out. Loose barrel nuts, loose castle nuts (allowing the stock to rotate and or the buffer retainer popping out and jamming the action), and more. The AR15 is a pretty simple weapon, but simple does not mean that you cannot mess it up. A factory built gun will generally be assembled correctly and you will have a warranty if there are any issues.
The best thing about the AR family of weapons is the massive amount of aftermarket parts. It can be overwhelming, and not all of it plays nice together. With more and more companies producing parts, they are not all interchangeable. For example, many hand guards now do not fit correctly on various billet uppers due to these aftermarket billet uppers using different dimensions then a milspec upper.
If you buy a complete rifle from a reputable manufacture, you know the parts they choose to use will work. If you build your own, you will need to do a little research. Sometimes trying to just buy all the best individual components will leave you with something that won’t work together. For example, some years back a few companies were making enhanced bolt carriers. A guy I knew purchased the LMT enhanced bolt carrier since it was supposed to be better than a standard one. He built a SBR with it, and found it didn’t work (The LMT carrier might not have been the only issue with it, but I’m using as an example). The LMT enhanced bolt carrier was tuned and built for a 14.5 inch barrel or longer. This guy just saw that there was an “upgraded” part, bought it, and never realized it wouldn’t work for him.
I could go on with more examples, but if you’re going to get an AR for serious use, or if you are not very familiar with them, it is recommended you buy a factory AR15 from a reputable company.