Try tun2socks if you need a functional IP stack with tun interface support.
The whole project is just some toy codes which compile but won't work. There are many known errors but they won't be fixed.
The initial goal is to create an IP stack accompanying NetworkExtension. However, I found it's just too difficult to create and maintain a correctly implemented IP stack, so probably it should be better to use a modified lwip or pico as the tun2socks library.