Being an IELTS and PTE instructor, I constantly edit and proofread essays. I use MS Word, with tracked changes and comments. Basically, my work involves: correction of spelling, grammar, syntax, punctuation, general improvements to overall flow and consistency, restructuring of sentences for greater clarity, language substitutions to avoid repetition, or to achieve a particular tone/style, and rewriting of broken or non-native English text to read as fluent.
By the way, some of my students were native speakers. They are good content writers, but not necessarily excellent proofreaders, because proficient proofreading requires good command of grammar.