I also use the terms “sandwich bag” or “freezer bag” if I don’t want to say the brand (ziploc bag).

Often you use brand names even if the thing you are using isn't that brand. Or just call it a zip-up plastic bag.

What Shelly said. Sometimes I just call them baggies. However most often I probably call them Ziplocs even if they’re Glad brand or other, because it’s already faster to say (2 syllables) than “sandwich bag” (3). And there’s a long history of brand names being used generically, at least colloquially, as others have noted.