There's no switch. The game starts with a tutorial, that transitions into the first campaign, which plays somewhat like a rogue-like. You lose the Falcon ship - you have to restart from scratch(FTL style).
But after beating the first campaign boss, the game transitions into a more open world setting, with a main spawn hub, where you(or any new player) can start fresh at any time. There's still a final boss(and with the latest update several Dreadnaughts bosses) present at that point but it's goal now is to destroy that main spawn hub. If it gets destroyed - it's pretty much GG. But there are ways to help defend it.
What most call MMO - is starting a server and skipping directly to the open world part. Where anyone can connect and start their own journey. Hence why it is used on the public server(s) that I know of.