you’ll either want to sample a cubemap, or use screen position uv’s to make it constant in size (though you’ll want to offset it based on view direction etc to prevent it from looking like a texture pasted to the screen)
i included a shader here on this forum post for a very similar purpose, not sure if it complies for urp and hdrp though
Question - Trying to Implement Multiple skyboxes in my Unity scene - Unity Forum