Learn Hardcore Japanese: kouzyou wo kengakusita ato, kaigisitu de situmon wo simasu. - After touring the factory, I ask questions in the meeting room.

Breakdown of koujou o kengakushita ato, kaigishitsu de shitsumon o shimasu.

Why is 工場 marked with を? It’s a place, so why not で or に?

Because in this sentence 工場 is the object of 見学する.

Compare:

So even though 工場 is a place, here it is treated grammatically as the thing being looked at or toured, not the place where another action happens.

Why is it 見学したあと and not 見学するあと?

Because あと in this pattern usually follows the past short form of a verb when it means after doing X.

So:

This is a very common grammar pattern:

So V-た + あと is the key pattern here.

What exactly does あと mean in this sentence?