How do I make a mutli-select field and in my template get all the possible values for it?

I have a grid of employees that can each have multiple “departments” they belong to. So I need to use a multi-select.

However, on the page, I want to have a dropdown that allows choosing one of those values, and then it’ll filter out the employees that don’t have that department.

I don’t see where i can populate the multi-select with a channel, and I can’t figure out how to get all the values for my dropdown in the template.

How would I do this?

ExpressionEngine 3.5.3