Why is it not possible to use 'partie' instead of 'sortie'. Partie appears in the hints too
Différence entre partir et sortir?
Partir means to leave, depart
Sortir means to go out, exit, go out (on a date)..
Why is it il est sorti but elle est sortie?
With the auxiliary "être", the past participle agrees with the subject, as adjectives do.
Because this form of Past Tense (Movement) has genders (depending on the person/people).
Look up maison d'etre verbes
hopefully this is not a bad question. Is it always sortir "de" and not just the verb followed by the noun?
Yes, although you can also have "sortir" by itself.
"je sors" - "I'm going out"
"je sors avec elle" - "I'm going out with her"
"je sors de la maison" - "I'm going out of the house"