BASE DEFENSE BUILDINGS:
Terran:
When Terran needs anti air defense he need to build engineering bay then he can place turrets.
Protoss:
When Protoss needs anti air defense he need to build forge then he can place turrets.
Zerg:
When zerg needs anti air he can place spore crawler without evolution chamber but still he doesn’t need to build evolution chamber because he has queen which is the always built core unit and base defense for both air and ground and is also required for zerg base to operate properly.
FIGHTING ANTI AIR COMPOSITIONS:
Terran:
The best unit against air targets in Terran army is viking but you need to micro them (it’s not hard but it takes time and prevents you from doing other things).
Protoss:
Same as Terran needs certain units and micro them (especially focus fire).
Zerg:
A) build corruptors and go a-move (you can’t really micro them but they are still better than other races air units even when you don’t micro)
B) get 2-4 vipers cast parasitic bomb and watch how enemy splits but still dies to insane damage.
FIGHTING GROUND COMPOSITIONS:
Terran:
Terran has to split marines, constantly siege/unsiege mines, tanks, liberators (all separately), sometimes casts EMP and also using focus fire on stronger and tanky enemies like ultralisks.
Protoss:
Protoss has zealots frontline which is a bit easier but still some micro is required and have alot of spells to casts (mostly storm).
Zerg:
A-move because most of the zerg compositions are melee units and you can’t micro them. Occasionaly cast a spell like fungal.
GAMEPLAY:
Terran:
Terran needs to harass enemy and make constant pressure then win the game as quick as possible - the best way to do it is with one big push in midgame.
Mech defensive style is viable only when enemy doesn’t make swarm hosts against you (they always do).
Protoss:
Protoss needs to harass enemy and make constant pressure in PvZ however in PvT early game harass into defensive style also might work.
Zerg:
Turtle until late game (early game indestructible due to queens), need to care only about one strong midgame push, then a-move in late game for victory (remax whole army in up to 30 seconds if necessary and go with the second attack while enemy is still weakened).
Some other quick facts: zerg is producing any kind of unit from one building, overlord “free” scout and is the only one race that has free units overall.