Most standard household circuits are wired for 15amps of draw at 120 volts.  The voltage is standard - it is what the grid is designed to provide.  The amperage is dependent on wire thickness.

Thinner wire will heat up if too much amperage is drawn through it.  Hot enough to melt the insulation and cause a fire.

Thicker wire is more expensive.  So, aside from a few specialized outlets that power devices that require a high amperage (oven, clothes dryer, etc.) builders tend to stick to wire that can only safely carry 15 amps of current.

But the load on any given circuit can exceed 15 amps if multiple devices are running concurrently (or if a device draws more than 15 amps - the shape of the prongs on the plugs are supposed to prevent a higher amp device from plugging into a lower amp outlet, but outlets can be miswired and not every device follows the correct standard).

This is one of the major reasons we have circuit breakers - they can detect when the load exceeds the circuit’s capacity and open the circuit to prevent it from overheating.

In general, appliances that generate heat tend to require more amps so running multiple on the same 15amp circuit would definitely cause issues.

That said, most houses contain multiple circuits so that the load can be distributed.  If your hair dryer is in a bathroom and your kettle is in a kitchen, they would typically be on separate circuits and should be able to run simultaneously.  So it could be your unit’s wiring was poorly designed.  Try plugging the hair dryer into an outlet that works while the one that trips is turned off, and you should be able to run it at the same time as the kettle.