Many good explanations here already. The phone nextcloud app does what you're saying, you can delete it locally from the phone (and be given the option to delete from the server too if you like) - on desktop app there isn't the option.
Conversely you can set the desktop app to auto-sync i.e actually download anything that gets put on the server, whereas phone app will always require that you manually download anything you want to have on the phone locally.
I agree that this is obvious functionality in both cases that is lacking, but go look at the github. People have been complaining about it for years.