Questions about example sentences with, and the definition and usage of "Onsite"

A: "On a typical Monday, I am not there, but I will be sure to be there next Monday."

Mondays is plural because they are talking about every Monday, not just one Monday. If they made Monday singular, it would mean the same thing, but would not sound as natural.

A:

It’s used when referring to a business, typically a plant or a construction project. On-site mean the person is at the plant or construction project. Off-site means they are not.