They are pretty standard. However, many will upgrade the trigger, safety, stock, grip, and even trigger pins. Those are all parts that you can buy piecemeal, either now or later on. It all depends on what you want on your rifle.For you experienced AR-15 guys: would you recommend a certain lower parts kit or does it really matter as long as it is built to mil-spec? Or would you recommend buying stuff piecemeal? I am very uneducated on AR builds so far, but from what I understand it's really the Upper that houses all of the stuff you want to splurge on? Aside from perhaps a more tuned trigger?
Any good step-by-step resources you could link to for a total newb would be awesome. I'm fairly mechanically inclined so I'm planning to mostly build from scratch and buy stuff in stages as to spread out the cost.
If you are looking for an upgraded parts kit, I would check Palmetto. They often have kits with better triggers and stocks.
https://palmettostatearmory.com/ar-15/ar-15-lower-build-kits-parts-kits.html
For builds, there are some good ones on youtube that will even tell you what tools you will need. I recommend buying a complete upper that is 5.56 (make sure) rather than building one yourself.