I've built a lot of them. The lower receiver is the easy part on an AR15, there's only a couple tricky parts like keeping detent springs from flying away and putting in the bolt catch pin. The rest is just like putting together legos. You don't even need many tools, just the stripped lower and a lower parts kit (LPK). Maybe punches to knock in the roll pin for the bolt catch and stake the castle nut, a spanner wrench, and a screwdriver for the pistol grip. Everything else you just push in with your fingers.
Uppers are a little more tricky. I've only had them apart a handfull of times and never built one from scratch. I have the wrench to do it but without a proper vise block you risk twisting the receiver when you torque the barrel nut. Then you have to worry about headspacing, getting the gas tube/gas block/gas port right, not mangling the gas tube, etc. I should learn and practice all that, but I never have.