Nested grouping is possible using legacy groups. These are group entities defined in YAML, not Group helpers defined via UI (which don’t produce a group entity but whatever is the common domain of the selected entities, like binary_sensor for motion detectors).
I have used this technique, for many years, to create nested groups of interior lights by room, floor, and entire house. It can also be used for motion sensors (thereby indicating occupancy by room, floor, building, etc).
FWIW, you may be interested in the following feature which will eventually be included in a future version. It applies to temperature and humidity of an area but maybe it will (someday) be extended to support motion detectors.
I have to migrate ~480 events from my Homeseer system to HA. I’m still learning my way around HA itself… so learning YAML (with all the trial / error, researching, asking for help, etc) would greatly (3 to 5x depending on the item) lengthen the time it will take… and the pain/headaches.
HA is already impressive to me (32 yr software career) but I’m surprised there its still missing UI based ‘basic automation constructs’ like dynamic groups and super groups.
You may also want to consider using Labels… no YAML required, can be targeted in actions, and (with the Spook custom integration) can be managed programmatically in automations and scritps.
YAML has the potential to reduce time and effort as well.
If you need multiple, similar entities, you can define one then copy-paste it to make additional entities, making small edits to each one. The same process via the UI can be considerably more onerous.