Youtube is certainly willing to sell you a subscription to background play capability.
Other providers offer background audio audio sources that can play while you work in other applications. Searches for these vendors are pretty simple: background audio that plays under other apps, for example. Free is good, and free is available.
Sound FX apps are abundantly available, particularly in the Apple environment. Many of them have built-in loops for sounds like trains, crowds, gunfire, etc. Most seem designed to be used with other apps.
LIVE audio apps exist for police scanners, which seem unperturbed about playing in background.
the Wayback Machine has abundant music Archives, manynof them featuring entire albums from throughout recording history, including radio and TV programs in addition to music of every genre.