"at college" vs. "in college"

According to Merriam-Webster, "at school" is an idiom which means

attending school as a student

From Cambridge Dictionary

I studied modern European history at college.

I met my husband when we were in college.

In the post "Hi Joe" vs. "Hi Mr Parker" when writing an email, I said

… By tutor, I mean he doesn’t teach at college or school…

Which, I suppose, is in accordance with the use of "at school".

Cambridge Dictionary indicates both "at school" and "in school" are in use though Ngram Viewer shows the former is less common than the latter.

Do they mean the same thing? Which one should I use?

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Note: I’ve gone through the post Preposition confusion – Do you learn somthing 'at' school OR 'in' school?, which is related but different. Because "teach at college" means neither the location or the process of attending classes