Does optimised battery charging turn on automatically?

Im charging my phone as normal as before and it goes to 100%. I do not turn on Optimized battery charging because i dont know it exist. Suddenly, when i charged it today, it stopped until 80% and when i googled theres optimized battery charging on iphone. When i looked into My phone settings and my Optimized Battery Charging is turn on. Does it can turn on automatically? I need an explaination. I just bought this phone.