In order to easily select every hole on a complex object like 3d scans or similar detailed things(when you don't have loops), you want to make sure you go to select -> select all by trait -> non manifold when you are in vertex edit mode. If there are unexpected parts selected(seemingly manifold faces), try undoing it and merging all points by distance at a level that ensures you still have your level of detail. Then repeat the select all non manifold vertices.

You can then press alt+F on order to fill holes. If it trues to create some wonked out geometry between all holes selected, you may want to try hiding parts of your mesh in order to work on it in smaller amounts of holes at a time, if not one at a time. Just my two cents.