Officially it supports linking other devices like their desktop app as a secondary. I currently use this to link into signal-mautrix on my matrix homeserver. This way I can access signal from multiple phones and multiple computers using a matrix client instead.

But you still need one "primary" device and it has to be a phone, right? That's different from Matrix where you can have arbitrary devices that are all on an equal footing.

Yes. And, annoyingly, when you only use Signal occasionally, these desktop sessions expire. And you have to link again. And when you do, you end up with a gap in your conversation history because "security".

Yes, and there's also a limit of max 5 linked devices.

You can use Molly to put Signal on multiple devices or you can bridge it into Matrix or XMPP, but you'll always need to run on one "main" device.