Hi,
I was using photos, then comme memories which was said needed phtos in a first place.
Now I have both, but I am wondering if I may remove photos, and/or if I should go back using photos which may be integrated some features of memories.
They are both propoted.
So my question, is what is the strategy with regards of the functions of those two similar apps.
Which should I choose ?
Nextcloud is an open environment where everyone can develop applications for. Therefore you need to differenciate between the Photos app, which is a core app provided by the Nextcloud team, and the Memories app, which has been developed by Varun Patil for Nextcloud. If the Memories app has the Photos app as a requirement, you need to keep it on your server.
In addition to what @j-ed said, you can disable (not remove) Photos, and Memories will still work, but you’ll lose the album feature in Memories.
You don’t have to make a definitive decision to “go back to Photos” because you still have Photos enabled, right? So you can switch back and forth between Photos and Memories as you like, i.e. use both. And if you think Photos is good enough for you, just uninstall Memories. If not, or if in doubt, just keep them both around 
I mainly use Memories because of its Android app, which is much faster for browsing photos or finding specific photos on my phone than the Nextcloud Android app.
Thanks a lot for your answers, it did help me.