Can I control who sees which fields?

Not by field - no. Anyone signed in who can open a record sees every field on it. There is a field restriction setting in the form editor, and the screen itself says it is not enforced. Please do not use it to hide anything sensitive.

By record - yes, and it is real. Access rules on a collection decide who can read and who can edit, by person, group or role, and can depend on a value in the record. They are set from the access panel in the form editor, and they are enforced everywhere the data is read, including file downloads.

So the way to hide sensitive information is to put those fields in their own collection, linked to the main record, and put access rules on that collection. Record-level rules are enforced; field-level ones are not.

One thing that is easy to misread: hiding a field in the form editor hides it from everybody, not from one role. It is a layout choice, not a security control.