she can also be inside washing the apples..
Where is ending "-sa" coming from? nothing in "Tips and notes".
It's -ja, but like most conjugations that start with j, the j assimilates to an S, Sz, Z, or Zs.
Has the "j" been droped ? Bent mossja az almákat . Indecatining the thired person.
With sibilants the j is replaced with the sibilant - ie sj = ss
Can also be "he" or "it". Or even formal "you".
Is this the same as "she washes the apples inside"?
Sure. Hungarian doesn't really make a difference between simple and progressive tenses.