Ground properties viz power and digital
Different grounds are a design choice. Depending on the expected currents and expected interference grounds are separated on chip and / or on PCB or even in a complete system (finished product). There are no rules to do it like this or like that to guarantee succes. A strategy is chosen at the design phase and that is implemented. Sometimes certain ground connections are prepared on a PCB and conmnected or  left open in the final design depending on tests. Experienced engineers usually know which strategy to choose in a certain situation.

You worry too much about these things but have too little knowledge to understand why to choose which strategy. Why is this so important to you to understand this all now ? Some things require experience, this is one of them. Join a team with experienced engineers and learn from them.