The 40-pin connector is 180° turned related to RasPi. So all HATs have to be mounted turned too - the HAT is not on top, above the Pi board, but aside. Therefore it's difficult to mount them.
When looking to all OrPi types, it looks like some are turned and some are same as RasPi.
...furthermore I just saw that the GPIO voltage is much different from RasPi standard - so all the boards available on the markets will not work:
2G-IoT have 2,8V
3G-IoT have 1,8V
4G-IoT have 1,8V