I would think this could be done completely in the browser using WebRTC?

This Flipflop project relies on the Iroh library for NAT/Firewall puncturing and external address discovery. Connecting two 5G devices, without a coordinating central server is possible, but very hard. Iroh simply relies on a server which you could self-host. Cool stuff, we stress tested it a while ago and it somewhat worked.

NAT connectivity is unsolved. My university lab conducted one of the bigger puncturing studies a decade ago [1]. Recently, we also bought 40+ or something 4G and 5G SIM cards in Europe for puncturing [2]. My master student traveled Europe by bus to get these local SIM cards. He worked day-and-night to connect these weird symmetric 5G NAT filter boxes and got it working. Simple sync collision and birthday paradox brute force... It all works sometimes, but insufficiently reliable for mass-adoption. You can't build a decentralized social media ecosystem on top. Like Iroh, best to use a central server. So then you have legal liability for your users. Unless an AI agent buys the server fully autonomously using Bitcoin, next years students got that working[3] :-)

[1] UDP NAT and Firewall Puncturing in the Wild, https://inria.hal.science/hal-01597969v1/document

[2] Mass Adoption of NATs: Survey and experiments on carrier-grade NATs, https://arxiv.org/html/2311.04658v2

[3] Self-replicating seedbox servers using programmable money, https://arxiv.org/abs/2609.05431