So first of all glad that you got to this core app, that is no where near what it should be, and second I was thinking on this a lot since I got my phone and put (with the help of mdt tool) UBports on it and now lets get to your list of questions:
- File manager is an important app on any OS so in mobile OS's also. Users need a good file manager because they need to move or copy things (folders or files). In an open source OS file manager can and must be a powerful tool (I think there should be some "geek"/"developer" options like show hidden files or read root folders or something but not limited to this) and if this is possible those features must have a warning on activation something like becoming a developer on android where you need to do something strange like tap a number of times on android version to become a "developer" and have access to more "geeky" features.
- Not necessary in this order but: copy, move, paste, delete, open known file types with default app for that file type(not to put user to decide each and every time this should be accomplished using a settings menu or asking user upon first use and by letting the user to change those settings later using a setting menu) select multiple, select all. Last two for copy, cut and delete purposes. I know some of them are missing from the current OS but I guess we all want a functional file manager so in a nut shell all basic functions are important.
- Moving (copy and delete the originals as far as now) files and delete files are the most I used until now.
- No move option, no select multiple/all option.
Hope this thoughts on file manager help you. And thank you again for working on this!