320px

FYI: A WCAG audit will check your layout at 320px width. That's not because mobile phones go that low by default. It's because people zoom in. You can use a 4k display at 320px width. Max resolution and used resolution are not the same.

It's why I like to talk about low resolution and high resolution designs, instead of mobile or desktop designs.

Design for flexible use cases, not specific devices.