Sort of. Each app has the equivalent of a folder. You could use the Files app to copy a directory tree of music files from a remote server, into the folder of a music-playing app (e.g. Flacbox, Nplayer). Then all the files are "local" to the app. This assumes the copy doesn't fail, e.g. several GB over a flaky network, as the Apple-native Files app can't resume a copy, and will abort on every duplicate.

> and then let itunes browse it, locally on the phone ?

The on-device music app from Apple conflates downloadable iTunes-purchased files with streaming Apple Music. It does not play local files AFAIK. But you can use several third-party apps to play music & video.

The challenge is keeping the device and remote server in sync :) Hence GoodReader, which does a decent job of file synchronization within the constraints of a non-Apple app. It can also play music and videos, but doesn't have all the bells & whistles of a dedicated media app like nPlayer. But then those apps don't have file sync.

While apps (e.g. media players or editors) can edit-in-place or view-in-place files within other apps (e.g. storage providers like GoodReader), this doesn't seem to support the ability to maintain a continuous mirror view / symbolic link on a directory tree from GoodReader into nPlayer.

Other apps for an iCloud-free iPhone: 2Do (CalDav sync), local scraper/search DevonThink 2 Go (Webdav sync), Secure Shellfish (local access to remote files via SSH), HereWeGo (formerly Nokia offline maps), iCatcher (podcasts), Codebook (password manager with native sync to Mac), Kiwix (offline wikipedia/stackoverflow).