Go to the libraries manager tab, right click and select Open internal libraries folder.
The b4xlibs are in there (with a b4xlib extension), copy whichever ones you are interested in to a safe place. They are zip archives, so you can open/unzip them with any archive program (7-zip etc.).
B4X - Rapid application development tools for native mobile (iOS and Android), IoT (Arduino, ESP8266, ESP32 and Raspberry Pi), desktop and server solutions. - AnywhereSoftware
github.com
You can always poke around in side the jar files with the Luyten and JD-GUI decompilers and Jadx can pick apart the apk files.