A modal verb is a type of auxiliary (helping) verb  that has no meaning on its own but it modifies the main verb, changes  its meaning and gives more details about the action.

Must  is a type of auxiliary modal verb used to express certainty, necessity  or strong obligation which doesn’t come from outside, it’s a personal  opinion. It also expresses prohibition in the negative form.

https://open.books4languages.com/english-a2-grammar/chapter/must-modals-of-obligation/