According to the official https://material.io/design/layout/responsive-layout-grid.html#breakpoints, the breakpoints should be as follow:

xs: 0 - 600
sm: 600 - 1024
md: 1024 - 1440
lg: 1440 - 1920
xl: > 1920

Yet currently, MUI has the following as default
xs: 0 - 600
sm: 600 - 960
md: 960 - 1280
lg: 1280 - 1920
xl: > 1920

Edit:

• The values were updated in https://material.io/blog/material-design-for-large-screens, the new values: https://material.io/design/layout/responsive-layout-grid.html#breakpoints