After years of open beta development the game had a controversial Steam release in March as a pay-to-access game that was otherwise free with their own launcher, now has relaunched as completely free.
The game uses the War Thunder engine and imports their models for a realistic theater of war with planes, tanks and infantry.
Many games on steam are purchasable/downloadable/installed through steam, and then default to opening up whatever launcher the game normally uses.
In some cases you can customize or skip this with an additional parameter on runtime, but for any game that requires you to be … some kind of logged in to the game’s own user management system, usually this does not work, occasionally it can get you banned by the game.
tl:dr; steam probably just launches the gaijin launcher that then launches the game.