"sä" can stand for "sinä" in spoken Finnish. The sentence decides its meaning.
Spoken form of 'sinä'. All of sä, sää, snää, sie and siä coexist; you will hear any of them.
sä, sää, snää, sie, siä
These are related spoken alternatives. Which one you hear varies by speaker and situation. Check each example rather than treating them as interchangeable in every sentence.
Mitä sä teet? - What are you doing?
sä here: you - spoken form of 'sinä'
Written Finnish: Mitä sinä teet?
Mitä sä otat? - What will you take?
sä here: you - spoken 'sinä'
Written Finnish: Mitä sinä otat?
Tuuks sä mukaan? - Are you coming along?
sä here: you - spoken 'sinä'
Written Finnish: Tuletko sinä mukaan?
Mis sä oot? - Where are you?
sä here: you - spoken 'sinä'
Written Finnish: Missä sinä olet?
Ei mitään erikoista. Entä sä? - Nothing special. And you?
sä here: you - spoken 'sinä'
Written Finnish: Ei mitään erikoista. Entä sinä?
Sä oot tosi kiltti. - You're really kind.
sä here: you - spoken 'sinä'
Written Finnish: Sinä olet todella kiltti.
Spoken and written Finnish are different registers. The examples show how "sä" is used here and pair it with the written sentence. Learn the meaning in context rather than replacing every occurrence of a word.