I am not sure what I have done is the proper way of doing it, but here is what I found to be functional.
The basic idea is to adapt the group members to allow the modifications by nextcloud, syncthing, and later any other program of file sharing.
$ sudo nano /etc/group
add to multimedia:x:1000:nextcloud,nextcloud__2
,syncthing,users,www-data
Save (Ctrl+o), exit (Ctrl+x), then use
$ sudo chown -R www-data:multimedia /home/yunohost.multimedia/share
This made the job for me, then you have your shared files in this folder for all users to use.