Do I need to transfer my sim card from my Android to my IOS iphone 12?

It’s the easiest way to transfer your number to the iPhone, but you don’t have to. You can get a new SIM from your carrier. Also, if you have contacts saved on the SIM you can copy them to the Contacts app on your iPhone from the Android SIM. This would be a one-time event; iPhones don’t store any data on the SIM.

If your current SIM card supports 5G, then yes. If not, then get one that does from your carrier.

Almost certainly not.

It may depend on exactly how you purchased your new iPhone- associated with a carrier, etc.

Also:

Move from Android to iPhone, iPad, or iPod touch - Apple Support