Learn Swedish: Han brukade flirta med alla. - He used to flirt with everyone.

You would move the object to the front, followed by a comma and a resumptive pronoun referring back to it. It would look like this: Alla, dem brukade han flirta med. (Everyone, them he used to flirt with). Notice that because dem is the object of the preposition med, it takes the object form, and the verb brukade comes right after the pronoun to satisfy the V2 (verb-second) rule.

Why do we use brukade here instead of använde for 'used'?

English uses 'used' in two very different ways: for past habits ('used to do') and for utilizing a tool ('used a hammer'). In Swedish, these are completely different verbs. Bruka (past tense brukade) is specifically for habits or things you usually do. If he were using a physical object, you would use använda (past tense använde).

Does alla here mean 'all' or 'everyone'?

Here, alla acts as an independent pronoun meaning 'everyone' or 'everybody.' When used on its own without a following noun, it almost always refers to people. If you wanted to say 'all of...' it would usually modify a noun, like alla vänner (all the friends).

I sometimes see flörta instead of flirta. Which one is correct?