To cheat in OTB games requires preparation and (sometimes) an assistant.
Recent methods include hiding a phone in the bathroom and checking it while taking a break. There have been instances where an audience member has provided signals based upon the “best” move available.
At least one former GM was accused of wearing a hearing aid that could somehow “hear” outside signals from a partner. I don’t think that she even plays chess anymore.
High level classical chess games take hours, and the players get up to walk around, stretch, take bio breaks and eat.
Plenty of opportunity to get outside influence if one wanted to.