move from Aperture to photos

You probably know that Photos is not a professional application. The transition to Photos will be smoother, if you prepare your libraries for the migration in Aperture, while you still have Aperture. If you still have to batch change metadata, do it Aperture.

And if you used hierarchical keywords, replace them by suitable flat keywords.

I posted a few hints, what to consider before migrating to Photos, when Photos has first been released. Notes on Migrating an Aperture Library from Aperture to Photos for Mac

Drag the App to the trash and empty the trash.

It might be wise to hang on to the app and its library lest you find that Photos does not meet your needs. Once you're sure it does, then by all means dump the app and its library.

Please tell, should I now DELETE Aperture altogether? I read that one should keep Aperture but not use it for some reason.

i would keep Aperture and the libraries on an external drive, until you are really sure that all photos migrated well to Photos. It will be difficult to get Aperture back, should you later discover, that a library has problems and you need to migrate it again. Do you have a backup of the originals you imported to Aperture? I am keeping all older Aperture libraries as a backup of my original photos, and I needed them occasionally to recover some image files that got corrupted. Some JPEGs simply get corrupted over the years by being copied between devices.

Very very simple. Just open the Aperture Library with Photos.

Done.

To do this: Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library'