"tosi" can stand for "todella" in spoken Finnish. The sentence decides its meaning.
"tosi" is also an ordinary written word. Check the meaning note beside each example; not every use is a shortening.
Sä oot tosi kiltti. - You're really kind.
tosi here: really - spoken 'todella'
Written Finnish: Sinä olet todella kiltti.
Te ootte tosi ystävällisiä. - You (all) are really friendly.
tosi here: really - spoken 'todella'
Written Finnish: Te olette todella ystävällisiä.
Kiitti, tosi kiva! - Thanks, that's really nice!
tosi here: really - spoken 'todella'
Written Finnish: Kiitos, todella kiva!
Kiitti avusta, sä oot tosi kiltti! - Thanks for the help, you're really kind!
tosi here: really - spoken 'todella'
Written Finnish: Kiitos avusta, sinä olet todella kiltti!
Mä tykkään tosi paljon saunasta. - I like the sauna a lot.
tosi here: really - spoken 'todella'
Written Finnish: Minä pidän todella paljon saunasta.
Mul on tosi nälkä. - I'm really hungry.
tosi here: really - spoken 'todella'
Written Finnish: Minulla on todella nälkä.
Spoken and written Finnish are different registers. The examples show how "tosi" is used here and pair it with the written sentence. Learn the meaning in context rather than replacing every occurrence of a word.