I order to dual-boot, you must install more than one operating system, plus a bootloader that lets you switch between them.
The multiple OSes cannot be installed within the same filesystem, which simply means they need to have their own area on the hard-drive/ssd/whatever.
For linux you need extX/btrfs/…, windows uses ntfs, and OSx uses apfs. They do not work with each other (or, well, linux can mostly access the file systems of other OSs but it can’t run from them).
In practice, this just means you either need more than one storage device, one for each OS, or that you need to partition your existing one. (Shrinking any filesystem already on it, and creating new partitions for the other OSes).
Essentially, imagine taking your 500gig mac, and shrinking it to a 300gig mac, and then using the extra 200 gigs to create a second, virtual ssd that you can then install your second OS onto while keeping the one you already have (though with a bit less space for it to use).
The “new” ssd is not really virtual, it’d be a partition of the size 200gigs, the filesystem that was already there was also a partition, it was just the same size as the ssd, so it took it up completely. When you only have one OS, you only have one partition (there are exceptions) but you can have two, or three, or any number you want. They can be any size as long as they together fit on the drive.