If you hold your finger on the AR15 trigger will it simultaneously shoot bullets?

No. The AR15 is a semi-automatic firearm. Pull trigger, it shoots one time. Pull trigger again, it shoots- one time. The M16 rifle is capable of fully automatic fire- pull trigger, it fires until you release trigger or run out of ammo. But that is different from the AR15. Note- in the US, fully automatic firearms (machine guns) have been strictly regulated since 1934. You cannot buy machine guns by walking into a gun shop, gun show, or on line. To legally have one transferred to you requires approval of the Federal government, a background check that includes photos and fingerprints, approval of your local Chief of Police, and paying a $200 transfer tax.