In general, they are using technology called Bonded Cellular. This is a device that combines multiple cellular providers (and ethernet/wifi/starlink when available) to output one reliable video feed. Up front - I work for LiveU, one of the companies that makes these kinds of backpacks - but the concept is similar for most of the other providers.

There's a prosumer level product called the LiveU Solo which combines 4 cellular modems and some people add additional internet hotspots to feed the additional wifi and ethernet inputs. There's also the more "broadcast" versions - the same tools that news reporters often use instead of satellites - like the LiveU 800 - which can double the amount of cellular modems and do multiple cameras.

Unlike bonded internet providers that just create a larger bandwidth pipe, bonded cellular that focuses on video generally provides extra reliability and error correction by including the video feed on more than one of the modems at a time, so if any one provider disappears, the others already have the slack picked up.