Basically, you run a server on your Mac that scans your music collection and "broadcasts" it to the network (LAN or WAN) via either the venerable UPnP/DLNA[1] family of technologies or the newish Subsonic API[2]. Of course, there are others, like DAAP or AURA, etc..
From there, you need to point a compatible player to said server to play your music on any supported device.
If you want to listen to your music on the go, pairing a Subsonic-compatible server on your Mac and a Subsonic-compatible app on your iPhone is probably the way to go. On the server side, I have only used the original Subsonic[3], which was good, but Navidrome[4] seems to be OK. But be aware that the whole "scene" is super messy and fragmented, with the usual abandoned forks of open source alternatives of everything.
Note that this means opening your local network, which comes with its own complexity.
This r/selfhosted thread[5] should give you an idea.
My use case is slightly different. I only care about streaming to my Denon CEOL mini system, which only supports UPnP/DLNA, so my current setup is:
- All my music is stored on a 2011 Mac Mini,
- I use Kazoo Server[6] (not perfect but reliable) to stream it to my audio system,
- which I control via the HEOS app provided by Denon.
Whatever stack you choose, make sure your files are tagged correctly and consistently.
[1] https://en.wikipedia.org/wiki/DLNA
[2] http://www.subsonic.org/pages/api.jsp
[3] http://www.subsonic.org/pages/index.jsp
[4] https://www.navidrome.org/
[5] https://old.reddit.com/r/selfhosted/comments/pz9dpb/lets_mak...
[6] https://docs.linn.co.uk/wiki/index.php/Kazoo_Server_setup_Ma...