I started reading bout tailnet and still have no clue what it is

It's a term they've had to invent because the term "VPN" has been twisted into being synonymous with "proxy server" nowadays, but really a tailnet is a VPN (not a proxy!)

A tailnet is a true "virtual private network" in the sense that it's a non-physically defined network to which which numerous devices can connect and see each other directly. The underlying physical network, a layer below, is (mostly) irrelevant to the operation of this network, and that's the part that's beautiful about Tailscale's implementation in particular. You could have a Pi Zero in your garage, a VPS in Australia, and your laptop in New York all joining the same private network ('tailnet') and interact as if they were on the same local physical network (in most respects).

Have a look at full mesh VPNs like:

https://github.com/cjdelisle/cjdns

https://github.com/yggdrasil-network/yggdrasil-go

https://github.com/gsliepen/tinc

https://github.com/costela/wesher

These build actual mesh networks where every node is equal and can serve as a router for other nodes to resolve difficult network topologies (where some nodes might not be connected to the internet, but do have connections to other nodes with an internet connection — I work with networks like that every day).

Sending data through multiple routers is also possible. They also deal with nodes disappearing and change routes accordingly.

tailscale (and similar solutions like netbird) still use a bunch of "proxy servers" for that. You can set them up on intermediate nodes, but that have to be dealt with manually (and you get two kinds of nodes). You also have to create routes for the node to be able to access the control server.