On a high level, I2P is an overlay internet on top of the regular web. Everyone has a router address that acts like a regular IP address, except that this one is purely inside the I2P software. So unlike IP addresses that go over your ISP to connect to the internet, on the I2P network your router can connect to other routers directly without the concept of ISP.
Your traffic makes multiple unidirectional hops over nodes in the network before it accesses the site/peer you want to connect with. Connection from your peer back to you goes back over another set of unidirectional nodes (unlike TOR where contacting and receiving uses the same set of nodes). The connection between the nodes uses the latest encryption methods of course.
For more details you would have to ask someone else.
I2P is a truly anonymous darknet where every user is a node in the network, unlike TOR where everyone is leeching off of the 6000 nodes. I2P also works great for torrenting. I2P is only for accessing I2P sites and not for anonymous clearnet browsing.
I have never tried the mobile version, but here’s some info for desktop:
There is a java version simply called “I2P” and there is a C++ one called “I2Pd”. Start with the java one, it’s easier and has built-in torrent webclient.
Install I2P from geti2p.net and start it. You are now a node/router in the network. To access I2P darknet websites like http://planet.i2p you have to tell your browser to use I2P proxy. You should use a different browser profile for using I2P, on firefox you can create one at about:profiles .
Enable I2P on firefox: Settings -> General -> Network Settings. Set manual http and https proxy to 127.0.0.1 port 4444 . You should now be able to visit eepsites (sites ending with .i2p). Always put http:// manually at the beginning. If it tells you to use jump services because it can’t find the site, just click on one of the suggestions.
Torrents are on http://tracker2.postman.i2p . Find one, copy the magnet link and go to the torrent webclient: 127.0.0.1:7657/i2psnark . Add the torrent there. Done, you are now anonymously torrenting.