I helped someone through a custom partition set up a while ago: Installing a Rocky 8.4 Virtual Box VM as a qcow2 Openstack Image - #10 by iwalker
That post has screenshots taking you through the GUI installer process step-by-step of setting up your partitions. Whilst this example was for a custom setup with a single partition, it’s easy enough for you to see and adapt it, creating partitions for /, /usr, /var or whatever other ones you need. You can even adapt it further using LVM instead of standard partitions - you’ll see how to do that once you get into the custom partition setup anyway.