Flag is not the right word for anything that connects stuff in kicad. They are called labels and power symbols (also known as power port)
In general there are 3 label types differing by their scope. Local labels are only visible in the current sheet. Hierarchical labels together with hierarchical pins can be used to define the interface of a hierarchical sheet. And global labels are visible for all sheets (power symbols are global labels.)
Yes you can create your own power symbols. See Tutorial: How to make a symbol (KiCad v5.1.x) (Section about power symbols)
Regrading the use of hierarchical design: Hierarchical or flat schematic design, what is best for me? (How to deal with multi page schematics?)