Is it true that there is a spot in the back of the vagina, that when I go to deep and hit it with my penis it will release bacteria and cause a uti?
You are probably referring to the cervix which is at the back of the vagina. UTI's are infections of the urinary tract and the urethra is not in the vagina, so it's highly improbable.
Welcome to A2A, gravel bone.
UTI stands for "urinary track infection." The vagina is not part of the urinary track, so "going deep" into her vagina should not lead to an infection. Also, keep in mind that you can't pass an infection unless you have one.
That said, it would be possible for you to pass a UTI, assuming that you had one to pass, if your penis rubs her ureter, which is entirely possible during foreplay. But remember, this is all predicated on you having an STI yourself and is not related to your penis entering her vagina.
Does this make sense to you?