Okay, so which structure is typically the site of fertilization? Keep in mind that fertilization is largely just down to timing. Often, what we see is that from the time an oocyte has been ovulated and entered the uterine tube and begun traveling down, sperm have entered the uterus and then entered the uterine tube and begun traveling up, and they just tend to meet in the middle, which would be the ampulla. There are some additional anatomical reasons as to why the ampulla tends to be the site of fertilization though.
The smooth muscle contractions of the uterine tube, particularly within the ampulla, tend to propel the egg and the sperm toward each other. We also know that the ampulla is the widest part of the uterine tube, 'ampulla' meaning an expanded structure, and so the egg and sperm literally just have more wiggle room to do their thing. The mucosal lining of the ampulla actually secretes a substance that can help with sperm viability and motility. For all of these reasons, the ampulla tends to be the site of fertilization, and there you go.