Specifically, is the belt reloadable or does the belt disintegrate?
Yes and yes. The ammo is held together by links that come apart when fired but can be re-used, often referred to as "distintegrating links".
I was a Marine machine gunner from '97 - '01. On our main weapon, the M240, the brass ejects straight down and the links eject to the right off the feed tray. The links are reusable as long as they're not too bent up, and you can make belts as long or as short as you want. Sometimes during night shoots we would take all the tracers out of a belt (one in five are usually tracers) and make one belt of only tracers, which looks pretty cool when fired at night.
The term you want to search for is "links". A quick search of both "5.56 links" and "7.62 links" turned up a bunch of sellers, both private and commercial.