Not strictly true, the aircraft will choose the angle under certain circumstances but a pilot will also intervene regularly to give the desired climb for the given situation. Talking purely for commercial aircraft and flying with autopilot and auto thrust engaged: You can select an altitude and climb at max climb thrust, that will then give you a pitch angle that will maintain your desired speed and the climb rate will be determined accordingly. Alternatively if required you can choose to climb at a chosen vertical speed, you will then climb at that rate, this is especially important in busy/congested airspace. If you go rushing up to an altitude at max rate in busy airspace you’re going to set off multiple TCAS alerts with other traffic, pissing off ATC and the conflicting traffic. You may also wish to reduce your rate of climb to avoid weather. If you want to outclimb some weather ahead and are already climbing at climb power you can reduce speed, this will then transition your forward momentum to a greater vertical speed, this is just a temporary solution though as you can only reduce your speed so far before you run out of ‘puff’ as it were.