Mine shared files are searchable same way no matter if I join any Channel?
Joining it only makes me visible to others in the channel, increasing number of people browsing my shares and possibly downloading?

My shares are browseable even when not joined a channel? Thank you

Yes, once your files are hashed they are available to anyone/everyone on the network.

If you want to restrict the shared files to certain channels, it seems to be possible if you configure the app accordingly, see:
https://support.fopnu.com/groups