Be warned that none of this is particularly easy to learn how to use as yet; the learning curve is steep.
Textures can be done directly in Blender already:
Landscapes & backgrounds:
Nvidia’s GauGAN is free for now, but complex: http://imaginaire.cc/gaugan360/
Quick intro and links of several (freemium) tools like it: https://samdavisphd.com/2022/05/ai-landscape-generator-how-to-create-artistic-masterpieces-with-ai/
You can also create landscapes and environments in Stable Diffusion, but it doesn’t specialize in it and the controls over it aren’t nearly as good as GauGAN.