These are the four basic grounding Symbols.
Figure 2.31 Page 44
Scherz and Monk (Practical Electronics for Inventors)
Is connect to ground through a metal pipe.
The earth has nearly infinite amount natural charge to safely distribute a charge.
Frame or Chassis
Frame or chassis grounds are when the wire is connected to the metal box. The box can either be connected to ground, or the entire frame is used as a floating return.
Digital grounds technically not do always connect to ground. They are used as return to complete the circuit witch is why they are often labeled.
Scherz, Paul, and Simon Monk, “Practical Electronics for Inventors”.
Mc Grall Hill, 2016.