As to the IoT-arrangement: you can hardcode the WiFi credentials in your own build, but it is also possible to modify them in the .uf2-file (uploaded to github) via Picotool. Copy the file to a respective directory, and use the following shell commands:

$ picotool config HIH4020-humidityMeasurement.uf2 -s SSID "WiFi-123"

$ picotool config HIH4020-humidityMeasurement.uf2 -s PASS "001122334455"

The downside with this method is that these data is now plaintext in the uf2-file and readable in a HEX-Editor.