To swap my mobile devices?

[Usedphone]

On a very superficial level, if I swap my SIM cards between my personal and work phone, would work be able to know?

The phone was reset to factory settings. And I don't really use my work phone like at all (but they gave me a much nicer device in comparison to my old personal one).

Depends on how your work manage the mobile devices. But we can see what devices are enrolled including version of ios and model.of iPhone as an example. So we'd know you'd swapped

Probably not

(Bear in mind that factory settings could be
true factory settings or a pre-built custom ‘factory’ settings but very unlikely)

It generally wouldn’t make much difference as long as no apps or device settings are required
But it could be an easy breach of policy on misuse of company equipment

There's no app installed, and we can use whatever Apple ID, but it does make me ponder. (I've never had a work phone before!)

No apps required! No VPN/SSO either....

Yes we can see all of it, doesn't require an app although usually something like Company portal. But would come through on the appleid management console although if you are using personal apple.ID that's rather odd so perhaps it's not set up as we have

Yes it's my personal one! For my work Mac it as a bit different (as I think there was the admin Apple ID and my personal one), but this is just my normal and personal Apple ID.

Well I would hazard a guess they are not managing this device then but have simply issued it to you for use. So they don't have any management or visibility of it. Terrible IT practice imo but there you go!
Ofc as PP said its also a misuse of company resources so depends if you're prepared to take that risk