The first paragraph at https://docs.mongodb.com/manual/core/write-performance/#indexes read:

After every insert, update, or delete operation, MongoDB must update every index associated with the collection in addition to the data itself.

Assuming the document doesn't move, only the indexes associated with fields that change (as part of the operation) get updated. Perhaps we can phrase it more along the lines of:

After every insert, update, or delete operation, MongoDB must update every index associated with the fields changed in addition to the data itself (for insert and remove operations, sparse and partial indexes may not be updated if the document does not have an index entry).

On MMAPv1, if the document moves, every index containing the document must be updated.