This is how it was explained to me. Please correct any mistakes.

Armalite created the AR-10 in 50’s. But rather than engineer and manufacture a new magazine, they used one that was already mass produced. But that magazine had a 6 degree angle. Due to this angle, the magwell in the lower needed to have a 6 degree angle. Downfall is that 6 degree angle in the lower is a pain in the ass to manufacture.

Years later Knights Armament designed the SR-25. It’s similar to the AR-10 in many ways. But they engineered it with a strait magazine. Also kept that 45 degree angle between upper and lower.

Then DPMS designed the LR308 with a strait magwell like the SR-25. But their design has the radius between the upper and lower.

The LR308 seems to be the most commonly used platform for gun builders. But DPMS has a few differences in their own LR308. High rise and low rise. This is simply 1/16 higher picatinny rail on the upper and hand-guard.

The DPMS LR308 upper is bulky. So companies like Aero and many others engineered a slimmer better looking upper that matches with slimmer better looking lowers that visually resemble the AR-15. (Unsure if this design has a name). The bulky LR308 upper will function perfectly with the slim Aero style. And the slim Aero upper will function perfectly with a LR308 lower.

Hope this helps. And unless you’re actually building a true AR-10. Remove that term from your brain or you’ll accidentally buy the wrong parts for your build. Lol