In the selection tools area, just below the options for Edge / Vertex / Polygon selection.
thee are a set of buttons which allow you to (Loop, Ring, select Between two selected parts, Invert the selection, and either contract or expand a selection. (+ / -) these allow you to Grow a selection to select more.
If you have more than one object in the modelling room, you can double click on any part to select the whole polymesh.
any other objects will not be selected.
As you work and build objects,.. it’s good practice to “name” objects and shading domains.
This makes selecting easier, as you can “Select by” name (polygon / polymesh / edge / Vertex).. or by shading domain.