ignore the opencv-python repo. that’s for packaging a redistributable.
git clone the opencv repo
then run cmake-gui, fill in the source path, which just the root of the cloned repo, let’s say opencv/, and the “build” dir, which can be opencv/build/ (will be created)
then run the “configure” step. if that looks good, run the “generate” step. if that doesn’t look good, dump that output here and ask.
that’ll include a BUILD_ALL target and an INSTALL target. the install target takes care to copy the proper files into whatever python was detected.