Usually either the Jellyfin/Navidrome app/web UI, or a native player app that talks to your NAS over the VPN.
For iOS I ended up building a small app for my own setup that streams files straight from the NAS (SFTP/FTP) over WireGuard/Tailscale so no media server in between. TestFlight if anyone wants to try: https://testflight.apple.com/join/PaQAsGcM
I use Manet as a music player an tailscale to have access to my home server. Before I tailscale set up, I’d just download what I wanted to my phone before leaving home.
For iOS I ended up building a small app for my own setup that streams files straight from the NAS (SFTP/FTP) over WireGuard/Tailscale so no media server in between. TestFlight if anyone wants to try: https://testflight.apple.com/join/PaQAsGcM