I recently happened to pick up a S&W MP15 lower for a really good price and I noticed on the website it says the lower is made of 7075 T6 Aluminum.
Is 7075 T6 Aluminum a mil-spec standard across the board amongst military, LE and enthusiasts as a quality metal in there lower??
I noticed there uppers were made of the same aluminum is this a quality metal or complete junk?
Welcome!
Don’t worry, a S&W M&P-15 lower is good and the metal in their stripped lowers and uppers is milspec.
ETA:
Now spend a lot of time reading all the stickies then buy quality parts to complete your rifle!
7075 T6 aluminum is TDP spec and therefore what an AR should be made from. 6061 T6 is the cheap crap you want to avoid. You did good.
What is TDP spec?
Do a search on technical data package if you want the full story but the short answer is it’s the military specs for equipment, in this case the M4 and M16. Rest assured that the metal in your lower receiver meets milspec. Now other things in your complete lower don’t but don’t worry about that either. (for now) 
You have a quality gun from a good company.