The grounding philosophy is simple, connect all metal parts of the vehicle to the negative battery terminal.
Some parts need to flex a lot and are connected with braided straps, other parts are not flexing so much and any cable can be used, finally metal parts are bolted together with star washers to make permanent electrical connections.
The size of the cables depend on the current requirements, the motor to frame must be heavy and flexible to handle the starter motor current as must the battery to frame cable.
The strap you are seeing may be the engine to frame connection and carries the starter motor current, an important connection.
A missing ground connection will not drain the batteries.