。
She finished doing her homework.
Literal
She [topic-は] homework [object-を] did-finished.
し終えた uses し (the masu-stem of する) + 終える to express 'finished doing.' This is the simple past completion — she finished it, full stop. The する → し stem change is important when building compound verbs with する-verbs: the masu-stem し is what attaches to suffixes like ~終える (finish), ~始める (start), ~続ける (continue). Pairing し with a productive auxiliary lets a する-verb tap into the same compound-verb morphology that godan/ichidan verbs use freely.