How well trained were the RAF during WW2?

They were very well trained. Probably the worlds best pilots overall, a little better or at least as good as the Germans. The German fighter planes were comparable but better than the RAF Hurricanes (2/3 of RAF planes), but only equal to or a little worse than the RAF Spitfire (1/3). German planes outnumbered UK planes 2 to 1.

So, how did RAF win.....several things in their favor.....

1.) Home court advantage, time in the air. Since the UK planes were fighting right above their own country and airfields they could stay up 1 hour or more. The German fighters were fighting at the ends of their combat radius and could only stay over England 10-15 min.

2.) Home court advantage, retrieval of downed pilots/ planes. Shot down UK pilots/ planes would fall over England and if survived would be used again (pilots, parts, planes when possible). When a German went down over England or the Channel, pilot, plane and parts were gone for good.

3.) The British had.. radar...crucial in the victory. British fighters always knew where German planes were going to strike so could be waiting to hit them wherever they showed up.

4.) Stupid German planning (Hitler again and also Goering). The proper targets for the German bombers were the RAF airfields and radar stations, taking them out, would have meant no places for RAF planes and no forewarning. Goering tried this awhile, even came close to a possible win, but then idiotically obeyed Hitler and switched instead to bombing London and UK cities, pointlessly killing civilians who could not have mattered in the battle anyway, while all the while, the RAF grew in undamaged airfields, more radar, more planes.

Well trained and prepared. Google Trenchards Plan for the Defence of London, and you will see the dispersal of the airfields was an important part of the strategy. The planes were better, much more suited to dog fights. Don't forget the advantage of fighting over home territory, and the crucial role of radar. The other thing that gets overlooked was that the Nazis became distracted by bombing London. Had they fully prosecuted the battle without respite it might have been a different outcome.

Btw Battle of Britain was the first exclusively aerial battle in history.

Not that well trained just had far superior aircraft with a less limited range than the Luftwaffe. That and you could reconfigure a spitfire by the side of the road , not so for a BF 109, took technicians to make field repairs. Simpler , not over engineered is much better at times.

Pilots at the peak of the Battle of Britain were sent into the air with as little as a half dozen hours' flight time in their assigned aircraft type. The only thing they had going for them was that they'd survived a couple of trainer airplanes that had been trying to kill them throughout their initial training periods (the British did NOT have a well-behaved trainer like the Stearman or the Texan). The Hurricanes and Spitfires, however, were not so inferior, and the RAF pilots when shot down were shot down over England, so many lived to fight again soon enough, and a bit wiser from the experience.

Very well trained.

It was a long battle,and both sides suffered serious losses, meaning that towards the end of the battle both sides were fielding inexperienced pilots.

The Luftwaffe abandoned the strategy of bombing airfields in Britain and targeted civilian centers instead. That was a serious error, as it gave the RAF time to regroup and recuperate.

They did it by preparing for it.

Look up the story of Chain Home. It was a case of placing fighter aircraft in position to intercept at short notice, not just the quality of the pilots.

There is a popular myth that the RAF was a bunch of gifted amateurs against a super efficient Luftwaffe.

However, the RAF was run by ruthless professionals. They had planned for the total replacement of front line aircraft and crews. There was an efficient system for repairing aircraft on airfields too. At the end of the BoB, the RAF had more aircraft and trained aircrew than at the beginning.

In contrast, the Luftwaffe had no such plans. Aircraft had to be returned to the factory for repair. They had no replacement system, so lost the war of attrition. They had actually lost before the first shots were fired.

They Luftwaffe also had a hero propaganda culture. Aces had to be protected. German squadrons had to protect the top aces, rather than hunt RAF aircraft.

On top of all that, the Luftwaffe was designed for tactical warfare. They had limited fuel reserves. The RAF were on home territory. They could rearm and refuel and return to battle.

Not trying to diminish the RAF's achievement, but Germany and its allies were planning on invading the USSR (Russia), and assumed that Britain would surrender because of the constant bombing of their resources. So, despite the overwhelming might shown by the Luftwaffe, the Germans were too busy looking east to really worry about an enemy they considered already defeated.

In other words, German short-sightedness thanks to the ease of conquering the rest of Western Europe in effectively weeks and months. They figured that Britain had to surrender (or more precisely, join the Axis) or be so devastated as to be helpless when they actually invaded.

It would have been different if the Germans had concentrated on "pacifying" Western Europe before even thinking about attacking Stalin.

No idea why you would think German aircraft were better build.