Did Japan send resources to help Germany and vice-versa? Were any European allies involved in the Pacific Theatre? It seems like they were entirely separate wars (Second Sino-Japanese War, Pacific War, Third Reich War), and that the Triple Entente existed in name only. I mainly ask because WW2 starts with the annexation of Poland even though the Sino-Japanese War had been going on far before, so it seems like WW2 was just Europe and then they said "oh yeah also America was fighting Japan" because of Eurocentricity
Oh my yes!
On the allied side you had Britain fully engaged in each theater, the fall of Singapore was considered a national tragedy equal to the Fall of France for the prosecution of the war. While the Royal Navy fought bitter bloody fights across the entire globe.
Imperial forces were also engaged. Australian troops first fought Rommel in the African desert at Tobruk and El Alamein before returning to the Jungle Hell's of New Guinea and the Solomon islands.
Indian forces were also present in Italy and across South East Asia.
Even smaller Allies were involved, despite his nations surrender in Europe, Admiral Doorman led a valiant rearguard allied force until his death at the Java Sea.
On the Axis side, prescious resources, plans, and even jet technology was sent back and forth by perilous U-boat voyage. While several subs operated out of the Dutch Indies known as the Monsoon Group, sharing Intel with Japanese forces.
Grandiose Axis plans also envisioned an eventual meeting in the highlands of Northern and Central India.