I was trying t code a simple chat app that worked p2p. So that generally means server-less, and is decentralized, no need for internet, just a local area network (lan).
After a brief search I found a few open source apps and also proprietary apps.
KouChat is open source and comes as a java app and is cross platform. Just to say this works very well, no latency. However it is very simple. Text and files, which is enough for a free app. I tested between fatdog64 laptop and android mobile. I got the apk from F-Droid. After installing, it updated itself.
Later I will take a look at TrueConf, CommFort, BeePEEP, LANcet Chat and others, and report the outcomes.