llegar vs alcanzar
Llegar is arriving. Alcanzar is reaching — by stretching, catching up, or achieving.
llegar
To arrive at a place or point in time.
Llego a las ocho.
I arrive at eight.
Arrival time. Llegar a + place.
Llegamos a casa muy tarde.
We got home very late.
No preposition needed with 'casa'. Llegar a casa = to arrive home.
Llegó a ser director.
He ended up becoming director.
Llegar a ser = to end up being. A result after effort.
alcanzar
To reach something by stretching, to catch up with, to pass something, or to achieve a goal.
Alcánzame la sal, por favor.
Pass me the salt, please.
Hand something to someone. Very common at the table.
No alcanzo el estante.
I can't reach the shelf.
Physical reach.
Alcanzaron al ladrón en la esquina.
They caught up with the thief at the corner.
Catching up. Also 'alcanzar un objetivo' = to reach a goal.
Tricky bit
In Latin America 'alcanzar' also means 'be enough': 'no me alcanza el dinero' = I can't afford it. Same word, new sense.