How is A/B testing typically administered for websites + mobile apps?

Hello fellow engineers!

I've read a good deal conceptually about A/B testing but wanted to ask how it is typically implemented in industry in the context of websites and mobile apps. To go off a simple use-case, suppose I have a React app and wish to test the effectiveness of rendering two buttons with different colors and call-to-action titles.

Are there any go-to services/libraries that specialize in A/B testing to help render the right components on the frontend and send/log results to a backend/portal or would I have to set up the rendering and analytics by myself? — Curious to know what industry practice is for A/B testing both in small-scale and large-scale settings; the more info the merrier.

Thanks everyone!