This is loosely based on https://github.com/schollz/raspberry-pi-turnkey, but in reality works quite differently. The underlying mechanism is more closely aligned with https://github.com/peebles/rpi3-wifi-station-ap-stretch.
An access point is set up on wireless interface uap0, which is always on. When this is accessed, the user can select which of the available WiFi networks the Raspberry Pi connects to. Once selected, the new network is joined without a reboot. The connection is confirmed in the web interface of the access point.
Run
./install.sh
You will be prompted for sudo rights. This will install required packages, set up the configuration files, and install a systemd service rpi-wifi-config which provides the web UI. Note that the script will overwrite some system files without prompting or backing them up.
The default hotspot SSID is rpi-wifi-config and the default password is 1234567890.
Once connected, the Web UI is available on port 7000, and the default IP address is 192.168.4.1. I.e. http://192.168.4.1:7000/
The SSID, key, and IP address of the Access Point are set at the top of the initialize.sh script.
The port for the Web UI is set near the top of the web_ui.py script.
This has been tested on Raspbian Buster running on a Raspberry Pi 4.