The other answers here are fairly uninformed. AI has been reliably beating humans in NHLE since 2019 with just deep learning [1], modern solvers can play within a small epsilon of a nash equilibrium (a perfect strategy) and are effectively unbeatable.
Referencing a solver while playing (what people call real-time assistance or RTA) is definitely a problem in online poker and is always prohibited. Solvers however play in a fairly predictable way and poker sites can detect if people are using them, though I'd imagine imperfectly. Saying that 99% of online poker is vs AIs is a hilarious overstatement.