ElementCriteriaModel combine with 'and' and 'or' filters
I suspect you're looking for this section on passing parameters to ElementCriteriaModels, in the docs.

The examples fail to highlight one aspect of the query you're building, but I think you'll find the same strategies hold up for many field values, as well as basic Element properties (like id, title, etc.)

For example…

craft.users({ myFieldA: [1, 2, 3] }).all()


…will fetch User Elements with the custom myFieldA matching the values 1, 2, or 3. The or is implicit!