You are confusing filesystem (organization) semantics with partition (storage) semantics.
Linux filesystem hierarchy is like a single giant tree with a stem (/) , branches ( /boot, /home, /bin, /usr, /var ) and sub-branches ( /usr/bin, /var/log ...). This metaphor is equivalent of the parents, children and grandchildren.
All these symbols/names in the filesystem represent points on the tree where storage space, like a partition , usb, external drive etc. can be hung ("mounted").
If you hang/mount some storage space only onto the stem of the tree (/) , then all the branches, and subranches of the stem (/boot,/home,/usr/bin) have to be contained within that storage space.
However if , after mounting the first storage space onto the stem (/), you then proceed to mount some additional storage space (e.g. another partition) onto one of the branches (e.g. /home) , then this second mounted storage is added to the total storage under the filesystem, but can only be accessed through its mountpoint (e.g. /home) on the filesystem. This second storage mounted on /home is in ADDITION to that mounted on the (/). All other branches of the / (like /boot, /usr, /var etc) will still have to be contained with the first mounted storage !
So / , /boot, /home, and others are simply access points on the filesystem. When you mount some storage onto any of those points (e.g. /), all the children and grandchildren of that point are automatically contained within this storage space UNTIL you mount some additional storage on one of its children or grandchildren.