Ctrl/Cmnd + Click on entity in viewport, adds/removes it from selection. Same in hierarchy.
Using Shift in hierarchy will select from last selected till clicked on, so to bulk select, just like in your filesystems.
Possibly if shift is held and holding in viewport could draw selection box to select many entities, but that's a bit later on.
Selection box is slightly separate feature on top of multi-selecting.
By the way, assets multi-selecting is already on prod, currently does not support multi-editing materials, but textures and cubemaps does.
Plus you can select few assets and see their total file size.
Deleting multiple assets too.