Skyboxes: (1 – 2 MB is ok)
https://polyhaven.com/
https://skybox.blockadelabs.com/
(Ctrl + U, search for _original and delete it)

3D Objects Library:
https://sketchfab.com/
(You can import GLB, glTF and zip files)
Note: Copy the Credits without Question marks!!! Save as text with the same file name and you find it easy.

E-Books:
Create virtual and mixed reality experiences in Unity e-book.pdf
The Unity Game Designer Playbook.pdf

Textures:
https://poly.cam/tools/ai-texture-generator

Blender:
https://www.blender.org/download/
https://www.blenderkit.com/

Linux:
https://manjaro.org/

Text expander Tool:
https://espanso.org/

Batch Image Converter
https://converseen.fasterland.net/

FileZilla:
https://filezilla-project.org/

Bildspeicher:
https://imgur.com/