Every category, both sides.
It adds up what each side sends and receives per game in every category: raw numbers for counting stats, and volume-weighted impact for FG% and FT%, since a percentage only matters as much as the shots behind it. You see exactly which categories you gain and lose instead of a single verdict.
Values are measured above replacement level, the best player usually left on waivers. The side that receives more players has to drop someone of about that level; the side that consolidates opens a roster spot it can fill from waivers. That is why depth is not free.
Yes. Connect your Sleeper or ESPN league in My Team, then enter a trade with a team in your league. The analyzer shows both teams' category ranks in your league before and after.