subir vs bajar
Up vs down — but also upload vs download, turn up vs turn down, get on vs get off. One axis, many contexts.
subir
Go up, rise, get on transport, upload, turn up volume or price.
Sube la música.
Turn the music up.
Volume, heat, brightness — subir for increasing intensity. Not 'aumenta la música.'
He subido una foto.
I've uploaded a photo.
Digital upload = subir. The metaphor of sending something up to the cloud works perfectly.
Sube al autobús.
Get on the bus.
Subir a = to get on/into transport. You go up into it.
bajar
Go down, descend, get off transport, download, turn down volume or price.
Baja el volumen.
Turn the volume down.
Reducing anything — volume, price, temperature. The opposite of subir.
Bajé la aplicación.
I downloaded the app.
Download = bajar. The data comes down to you. Very natural, very current.
Baja del coche.
Get out of the car.
Bajar de = get off/out of transport. You come down from it.
Tricky bit
Subir and bajar both take direct objects for physical things you carry: 'sube las maletas' (take the bags up), 'baja la basura' (take the rubbish down). And prices: 'han subido los precios' (prices have gone up) — the prices are the subject. Never 'han aumentado los precios subir.'