Minecraft on a Mac is always Java based.
Unused RAM is wasted RAM. macOS (and Windows for that matter) allocate a set amount of RAM to itself in order to run smoothly. Windows usually allocates half, whilst macOS 2-8GB depending on the amount of available RAM.
If your usage needs the 24GB, macOS will notice and allocate less RAM for itself.
Again, unused RAM is wasted RAM.
These are notifications from your browser. Go to the notification settings and disable all and any notifications from Safari that you don’t know what they are.
Your Mac is fine.