There are significant differences in plane durability. Some do seem to be much tougher than they should be and others are quite fragile(although it's generally accepted that most of these are realistic). At the same time, some weapons are simply more effective than others. Low tier British are really fond of the 7.7 mm machine guns... which are TINY and quickly become ineffective, even when you mount 12 of them on a plane. The 12.7 mm (.50 cal) machine guns that the Americans are fond of are significantly more effective. And once you get into the second era, it's when cannons become much more common. Almost everyone has at least one plane with 20mm cannons by era 2, other than the British(They might have Spitfire for it by this point, I forget the exact position on that one) and the American (Who don't get any sort of cannons till Era 3, I believe, and then it's the 37mm which is a different, although still very effective beast.)
To summarize. 7.7s will have a better chance of eventually weighing your plane down with enough lead to make it fall rather than actually damaging it. 12.7s will have a decent chance of critical damage to parts, and pilot, but rarely cause catastrophic damage to cause your plane to just fall apart. 20mm and 37mm cannons are what cause your plane to just disintegrate in mid air.
Russians are tough. Americans and Germans are still reasonably tough, but generally less tough than Russians. British are a little tough, but alot of their planes fall apart pretty easily. The Japanese made their planes out of paper... and then soaked that paper down with fuel... and then handed out free incendiary ammo to all of their opponents... and then taunted "hit me if you can!!!"
Very little of this game is P2W... there are a few premium planes which are generally considered a little overpowered, but you shouldn't be encountering any of them at this point. (emphasis on "shouldn't" It's been a while since I played, and the MM was a little messed up then and things that "shouldn't" be happening were.)
The best thing you can do is not put yourself into a position to get shot. Sometimes that's hard, sometimes it's impossible, but that's the only real defense against getting shot down. Being rammed is also an annoying thing to happen, and it does happen for everyone, but it's risky for both involved. There's a fairly famous .gif of a Japanese plane ramming a catalina and shearing it's wing off and the Japanese plane survived. That should basically never happen due to the structures of both planes, but it did, at least once. Russian planes can get away with it more often, because they're poorly modeled by a seperate team from the rest of the planes. But if you get rammed, the general outcome is that both parties will be having a very bad day. Exception being extreme tier/structure differences. A biplane ramming a b-17? B-17 might get a little bit of annoying damage, if the ram is in a bad place, but odds are it'll survive while the biplane wont.