Ugh.
Bethesda broke Fallout 4 for Xbox and never fixed it. The problem is largely with mods, but the base game can irreparably break, too. And modders apparently can't fix it. They've shown Bethesda what needs to be done, and Bethesda has ignored the issue. They may have come around to fixing it over the last couple years, I'm not really sure, but for a good year after the "next-gen" patch they were happy to leave it broken.
The issue, as I recall, is with the random generation of NPCs. It can break. And when it does — so, you get settlers that have a random chance to spawn in. Also raiders are randomised. A few other things. And if this combination of configuration choices comes together in a specific way, you can never approach this NPC. If you fast travel to them, the game will crash. If you walk up to them, once you get to within a certain distance, the game will drag down in FPS and eventually crash.
People complained about this for weeks and Bethesda deflected. Modders figured out the problem and told them how to fix it. They said no.
What's messed up is, if this problem isn't fixed, you may play through the game just fine. You may put 30 hours in and hit the bug. "Load an earlier save" you say. Okay but you don't know when the NPC was generated. You have to go back before then.
The issue was so bad that some storefronts (notably, GOG) refused to carry the next-gen patch. The game was fine without it, on PC anyway. (Macs can also play it, albeit with some emulation trickery.)
Buyer beware.