yea because the footer is position fixed, it is not part of the content flow, meaning it doesn't make the page height taller anymore.
that is why you could set
and 600px needs to be the height of the footer.
look at this website for example (not a lay theme website but the same principle applies):
https://100k.studio/
it has a fixed footer and the div above the footer creates space for the footer by having a margin-bottom
Screenshot 2023-10-18 at 12.04.36.png
if you cant make it work please post a link here