I got an NRF52840 dongle at Christmas and have been trying to work with Zephyr on it. An initial set of examples is over here. These examples include a simple “Hello World” with a blinking LED, a minimal BLE example and an example that drives an ST7735 display.
A couple of points about the wiring:
The ST7735 has a built-in 3.3V regulator so it is supplied from the 5V VBus pin.
The serial interface is connected to a USB-Serial converter (not shown). The TX pin (from the NRF) is on P0.20. The RX pin (in to the NRF) is on pin P0.24