Watching a lot of documentaries
Did Germany taking on Russia, cost them the second world war?... Why did he suddenly take on Russia ? with so many other fronts he was fighting on....
1) Germany had just conquered Europe. All of it. In weeks. With hardly any losses. Compare that with WW1, where ~ 2million German soldiers were killed and Germany lost the war. Clearly Germany in WW2 was vastly more powerful than Germany in WW1.
2) In WW1, Germany beat Russia repeatedly. Almost every major engagement was a German victory after initial losses in the eastern part of Germany (which Germany soon reversed). And Germany was so vastly more powerful now, as proven by the comparison above.
3) The USSR had just engaged in a full on war against Finland, which it outnumbered ridiculously. In tanks and planes, which Germany knew very well were key factors in war at that time, the USSR outnumbered Finland almost 100 to 1. Finland was completely isolated and couldn't even get guns from elsewhere, let alone any significant support. They were on their own, fighting a USSR with ludicrously superior numbers. The USSR barely won that war.
Given that information, which was current for that time, it was "obvious" that the USSR military was crap and the German military was superb. That also fitted in nicely with Htler's preconceived belief in German superiority. So it was "obvious" that Germany could defeat the USSR, probably very easily and quickly. So why not attack Russia, defeat it quickly and free up Germany to concentrate on other areas while also freeing up Japan (Germany's best ally) to concentrate on other areas as well? The USSR would probably surrender before Christmas.
We have the benefit of hindsight to let us know that the USSR was a lot more powerful than it appeared to be and bitterly determined to resist invasion at any cost, any cost at all. But Hitler didn't. It wouldn't have seemed as stupid an idea to him as it does to us.
Did Hitler purposely let the soilders at Dunkirk escape?
Maybe, but I think probably not. It's possible on the basis that Hitler would have preferred the UK to be a subordinate ally like Italy or to surrender and there's evidence that he thought those were possibilities at that time. Japan and the USSR were effectively allied with Germany, all of Europe was either under Nazi control or under the control of subordinate allies of the Nazis, the USA was staunchly isolationist and the British Empire was very spread out and much less strong than it had been. Also, there was a fascist movement in Britain itself. It wouldn't have been insane at that time to think that there was a possibility that the UK would become a subordinate ally or surrender.
But I think the key reason was lack of information, lack of cohesion in the German military and the screwed up political situation of a wildly egotistical dictator with people in various positions trying to gain his favour and thus power. The famous blitzkrieg wasn't really planned as such (that name was given to it later, by a newspaper IIRC) and resulted in a bunch of seperate units under seperate command in places they weren't exactly expected to be, not working as a whole and with contradicting orders.
Hitler didn't actually issue the halt order. That was done by one of the (too many) German commanders, who didn't really know what the British forces were and didn't really know what the effective German forces were either. Which ones were available for Dunkirk and which ones had already been sent to the advance on Paris? Someone would know, but who? Some units were getting contradictory orders. Which one had they obeyed?
As for the overall question of whether Germany could have won WW2...I think maybe
if Britain had surrendered
and Germany hadn't invaded the USSR. Maybe. Maybe they could have gained more resources and more advances in military technology if they hadn't been fighting Britain and all the other forces that joined Britain (India, Australia/New Zealand, Polish exiles, French exiles and French imperial forces, etc). Maybe they could have taken control of the middle east and wouldn't have been so short of oil for planes and vehicles. Maybe they could have fobbed off the USSR for long enough to gain enough of a technological edge to defeat them, maybe in combination with Japan attacking the USSR from the other side. Maybe the USA would have remained isolationist after a British surrender and maybe then Germany could have defeated the USA later on, after having developed more advanced weapons. Maybe threatening nukes from south america, for example. Maybe. Possibly.
I have a recommendation for a Youtube channel for you. Military History Visualized. It's mainly about Germany in WW2. He covers other aspects of military history, but that's his main area.
https://www.youtube.com/channel/UCK09g6gYGMvU-0x1VCF1hgA