Nice small piece about backtesting: https://www.priceactionlab.com/Blog/2020/12/misconceptions-backtesting/

Take infos on that website with a grain of salt. It's correct to be carefully with backtests, but all the rest, like "Type II error" or the "2.5% / 95%" table is fictional. If you earn your living with a trading blog, you must daily produce this sort of filler material.

There are serious articles about that topic, one here: https://www.factorresearch.com/research-quant-strategies-theory-vs-reality