Typical grid-tied solar system would still require a transfer switch, either auto or manual, they're called break-before-make transfer switches because they disconnect the mains supply before switching over to the battery + inverter system or generator. These transfer switches are mechanically interlocked so it's not possible to have both supplies connected simultaneously as this would generally be release-the-magic-smoke catastrophic.

Typical auto transfer switches are fast enough that computer equipment etc won't notice.

Edited to add: if you want truly uninterruptible power then you should do what data centres do: everything runs off the UPS all the time (well, all the computers, maybe not all the cooling system), the only thing that changes is the supply: either mains, batteries, solar, or generator. Or some combination of batteries + the other three.