Cual es la diferencia entre ser absolutamente seguro de algo y adivinar?

"Supongo"

Esta frase es más como "no me preguntes, ¿qué sé yo?"

Nos da la sensación de que la persona está admitiendo la derrota y está menos segura.

seguro de (hacer) algo; estar seguro de obtener algo o estar seguro de que algo sucederá

Estar absolutamente seguro significa estar seguro de lo que siente, lo que quiere o lo que ha decidido

Explanation:

ah, so ustedes and nosotros are my 'go-to' if i am using spanish mainly in the states? interesting, but still slightly confusing, haha."

it seems that you are a little confused about subject pronouns. yo=i, tú/usted=you, nosotros=we, ustedes=you(plural), ellos/ellas=they.

in the case of vosotros, it is an informal replacement for "ustedes" in spain. similar to using "you all" in english.

even though it is not generally used in conversation outside of spain, it is used many times in literature. i would suggest having at least some familiarity to that form, at least to recognize what it is.