How do you test mobile layouts across different screen sizes?

I spent months tweaking copy trying to fix mobile conversions, only to realize my main button was totally pushed below the fold on smaller screens.

Turned out IPhone 12 and smaller users couldn't even see the without scrolling.

I only caught it by accident on a friends phone since it looked fine on mine. Now i'm wondering how many basic design bugs i'm missing just by testing on my own device.

Do you manually check your site on older phones or rely on tools to flag this?