Q: If I buy a tablet, will it come automatically with an Apple Sim in?

Read this Apple Knowledge Base article...

Activate cellular data service on your iPad Pro, iPad Air 2, and iPad mini 3 or later - Apple Support

In the USA all Apple cellular+wifi iPads now ship with an Apple SIM included.  There are carriers who do not participate in the Universal Apple SIM program, so if you were to use one of those carriers you would need to remove the Apple SIM and substitute a nano-SIM from the carrier you wish to use (eg. in the USA, Verizon does not participate in the Apple SIM program, so to use Verizon you would need to acquire a Verizon nano-SIM and substitute it for the included Apple SIM).

Also, some carriers do participate, but once activated they lock the SIM to their network - for example AT&T in the USA does this.  Note this locks only the Apple SIM to AT&T, not the device itself.  It just means to change carriers you would need to acquire a new Apple SIM or use a carrier-specific SIM.