[ArnoVW]

LEGO Mindstorms is great, but it's amazingly expensive.

They recently released LEGO Boost, which has some limitations wrt Mindstorms, but you won't notice it unless you're doing advanced stuff. Moreover, it has a great app. And it's one third of the price of Mindstorms.

[cben]

Due to price, when you buy motors/sensors/hubs, you'd want them to be compatible with future buys. LEGO used to have several systems but now pretty much converged to one "Powered Up" interface: https://brickarchitect.com/powered-up/.

The app is indeed neat & powerful. Performance is limited by official firmware being "dumb", it reports sensor values and takes commands, all computations actually run in the app. However, what's cool is an essentially unbrickable bluetooth bootloader letting you try alternative FOSS firmware — see https://pybricks.com/.