i just did this starting last week...
Adding mods to the Xbox is VERY easy, start by creating an account on bethesda's website.
here is a link to the cheat terminal: https://mods.bethesda.net/en/fallout4/mod-detail/911793 click the "add to library" button it will add it to your list, google searching the name of the mod you want to add and include xbox one on the search and you should find the bethesda mod library page to add it to your library.
once you launch the game there is an option called "mods" and you will log into the Bethesda page /w the same UN/PW as what you created the account, once you're logged in you choose to "download" from the website.
Here is a list of a few mods that I have used up to this point:
1. cheat terminal: it's a holotape that shows up in your inventory and it gives you the ability to level up, turn on god mode, respawn NPCs, add caps, etc.
2. Unofficial Fallout 4 patch - i don't have this one working because it requires that you have all of the DLC content which I don't have but it fixes some of the dynamics/physics in the game
3. Sim settlements 2 - this is a whole DLC in 3 chapters, the characters are fully voiced. Basically in the first chapter you get introduced to a character who has a new tech that automates the ability to build new settlements, the inital levels walk you through how to do everything and as your settlements grow you gain the attention of some less scrupulous people who want to rain on your parade.
4. America rising 2 - brings the enclave back, i haven't started this one because I picked up a save where I had already beaten the game and this one introduces a whole new faction and gives you the opportunity to have a new ending with that faction so I will probably have to load a fresh game and add this mod.
There are dozens more out there, the xbox is hard coded at 2GB of mods loaded at any one point in time. so choose your mods carefully and recognize that you likely will have to enable/disable to experience the various different stories in the mods.
So far the only cost I have incurred besides a Microsoft game pass subscription is a $4 DLC to enable one of the mods and I have probably added 15 hours of unique content that has been fun and engaging.