Just because a journal is open access does not mean it is predatory. In fact, many legitimate journals will charge article processing charges (APCs) in order for research to be openly and freely available. Especially in the sciences, this ensures that authors meet open access requirements of governmental and other funders. See the 2022 Office of Science and Technology public access policy, for example.
Predatory journals are unique in that they do not employ best editorial and publication practices, not because they charge publication fees.
Not necessarily. It is just not likely that the research has undergone rigorous peer review. Therefore, we are less confident in its quality.
Traditionally, scholarly journals have a peer review system in place to ensure the research they publish is of high quality.
Predatory publishers are entities that prioritize self-interest at the expense of scholarship and are characterized by false or misleading information, deviation from best editorial and publication practices, a lack of transparency, and/or the use of aggressive and indiscriminate solicitation practices (Grundniewicz et al., 2019).
And while publishers can and do make money off of the scholarly content they sell to libraries, business, etc., they also aim to contribute credible research to the scholarly record. In contrast, predatory publishing:
Over the years, whitelists and blacklists have been created in an attempt to help authors distinguish between predatory and legitimate journals. However, because each journal is unique and editorial practices can change over time, it's best to evaluate each journal on an individual basis using some established criteria.
If you receive a solicitation from a journal and are unsure whether it is predatory, talk to a trusted professor or librarian.