One step in installing nixos I had trouble performing was connecting to wifi. On the minimal install boot wpa_supplicant is installed but there is no service. There are a few instructions for this online but this is what I did to get it working.
nano /etc/wpa_supplicant.conf #this file can really be anywhere
Once the file is opened add
Then start wpa_supplicant for your interface.
wpa_supplicant -B -i iwp3s0 -c /etc/wpa_supplicant.conf
Once wpa_supplicant is running on your system you can add a network through the cli.
wpa_cli scan scan_results add_network set_network 0 ssid "..." set_network 0 psk "..." enable_network 0 quit
Next test the connection.