How do I tell whether the Volvo XC70 actually needs maintenance or it's just the vendor attempting to earn more money? How do I self repair it and diagnose if I don't want to go to the vendor?
Vehicle maintenance is based on time and distance travelled, so maintenance tasks are completed at particular distances or times.
At 5k, it may be change oil & filter,check brakes,
At 10k, it may be oil & filter, fuel filter, air filter, check brakes etc.
So there are time based tasks, such as checking anti-freeze concentration prior to winter and changing the brake fluid every 2 years (hygroscopic).
At any check if something is found wanting, then it should be corrected. Any "new" noise warrants checking it out.
The examples above are just an idea, not specific to any vehicle, Volvo will have produced a detailed maintenance schedule for your car, which you should follow. Searching gives many links, this may be a good place to start
The ecu (computer) has more recently been programmed to warn owners that x, y or z needs to be done. Some owners used to drive vehicles until they broke down, ignoring any interval so the manufacturers have started dealing with this.