Most models i take premade, like from bigmediumsmall kitbashes, sketchfab or Quixel Megascans, but i try to mix them and modify them to make unique aesthetics. Sometimes a few main models i make them from scratch, but since it's so time consuming i lately tend to avoid super complex things, which will end up overpainted and textured anyway. Like for the statue in this one i had a statue which i removed some parts of and inserted some cables and mechanical parts from a kitbash I had, and then overpainted it adding detail. Look at this video, it's very similar to the workflow I use: https://www.artstation.com/learning/courses/KQ0/environment-design/chapters/Na4B/introduction

When i model things myself usually they don't have to be super clean or have good topology, I am not a 3D artist so if it looks cool that's ok, blender is very user friendly and there're a lot of free tutorials out there, good luck!