Creating Games like Mass Effect 3, KOTOR, etc...?

If you want the best results, then yeah, you need to create your own custom content. And without much training in development you’d struggle to be able to use pre-made content. Ultimately, making that type of game by yourself is a huge amount of work, it took them years with a group of experienced developers so consider how long it would take a single person or a small team of novices.