Will I be writing 1 essay for each college AND a common app essay?
Commonapp essay and specific school supplement essays if they have them. Check under “questions” too because the school might put it there.
Usually, yeah. Most individual schools will have their own supplements (often several), but they’re usually only a paragraph or so.
It varies a lot. More selective schools often have one or!more additional essays with specific topics (often “Why College X?”, but not always). Within the Common App the college supplement has them. Some colleges release their supplements a bit later than the date the Common App opens, though.