Nano 33 and mBedOS

I read that the Nanos 33 came with an embedded OS (if I understood correctly).
Does this change the usual Arduino way of working - write / compile sketch in IDE, followed by upload ?

No. You can use the Nano 33 BLE just as you would any other Arduino board.

You can also Mbed OS API to program your Nano 33 BLE, either directly, or in your Arduino sketch using the mbed namespace using the Arduino IDE as usual.

An example how the board is different is that on other Arduino boards if you call delay(), the microcontroller just spins waiting for the delay to finish. The Nano 33 BLE sleeps during that delay. So you can actually reduce power consumption by using delay(), which is the opposite of the situation with the classic Arduino boards.