Are you tiling textures? If not that's what you'll want to do; to be more exact modular pieces + likely trimsheets and maybe vertex blending to add variety, if not decals. You can start by reading the polycount wiki and the checking resources listed, then search those terms to learn more. Modular Environments Texture Atlas…
@Yeudielcc That's the spirit! Now you know the names you need to search you can find a lot of material and process breakdowns on these, including here on the forums. A quick search on trimsheet turned up this video, which I just sped through but seems good, being the first of an in-depth series explaining the hows and more…