you've probably noticed already that every trading strategy, ever, is defined by the close price. It's always "closes above this level" etc.
in crypto and forex, any 24hr decentralised market there's no close as you said. My chart might show a daily candle as between midnight to midnight GMT, but someone in a different timezone that'll obvs be a different time, and a different looking 'close'. Also there's no gapping in 24hr markets, I'll be covering that also as most candle patterns are for stocks that gap...
You could say that in stocks though there's a defined close if the stock only trades on one market, like on the LSE or something, so a 'day' must be between 0800 and 1630. But even then some have extended hours to 2100 and a pre-market open...