Method 1: TuneFUSION

Install TuneFUSION app on your PC with access to your music library, configure it.
Pair with foobar2000 mobile's TuneFUSION client to begin automatically syncing.
Newly added tracks will be copied to your foobar2000 mobile library per TuneFUSION settings.
In order to finish initial upload, with gigabytes of music to transfer, stay on the "Sync with TuneFUSION" page in your foobar2000 mobile app - it will prevent your device from going to sleep until sync has finished. You might want to connect your device to AC power to ensure uninterrupted operation.
That's all.

Method 2: FTP

Connect your device to your wifi network.
Open foobar2000 mobile.
Go to Tools (gear icon in the corner), Utilities section, FTP Server.
Use common sense to configure. If your wifi network is not private enough, set a password.
Allow write access in order to be able to manipulate your music library from your FTP client.
Toggle "server running" to start.
Enter the ftp:// URL presented by foobar2000 mobile to FTP client on your PC. I use FileZilla for this purpose, but Windows Explorer should work as well.

Android: on some of modern devices the default privileges won't let foobar2000 modify device's music folder contents, only foobar2000's private music folder. To work around, choose "share specific folder" and choose your music folder. Then you can manipulate its contents.

Method 3: UPnP

Explained in detail here: