If you are talking about the built in gnome integration it should show up on the left side of the file chooser popup. Sometimes applications can’t seem to open this folder until you manually click on it in the files app. As far as the location on the disk it appears to be somewhere under the /run/user/1000/gvfs directory.
@lo0
Thanks
I have the Open shot video program both from the software centre and an appimage
the one I install from the software centre finds the Nextcloud and the files in it
but if I use the AppImage [which is newer] it cannot find the Nextcloud folder.