As no one answered this, i give it a try.
You need a 3d program like Blender. You need Photoshop for the textures. You need Unity, you need some skills in programming, as just replacing the existing 3d model will not work properly (it uses the games texture) and you need a lot of time. And you need to get used to the "inspector" for finding the right assets in the game you want to replace. As a start i suggest you read the fandom wiki for modding uboat.
Note: Yet no custom textures are possible when replacing 3d assets. So if you create a nice 3d model of the torpedo room, it will use the existing texture and that leads to an odd look. All new 3d elements will only get the existing texture and there is not space for new texture elements on it. You might give it a try, but i guess its frustrating as to now.
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