When it comes to alternatives to the clearnet usually the name that pops up is Tor or some similar software. I think I get what Tor roughly is but both i2p and Freenet are still a complete mystery to me despite my Google searches and questions to ChatGPT.
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i2p is basically just tor, but without exit nodes. you are intended to access sites within the network only. here is an eli5 of that https://www.reddit.com/r/explainlikeimfive/comments/ypbj0c/eli5_what_is_i2p_and_how_does_it_work_how/ but it uses “garlic” routing https://en.wikipedia.org/wiki/Garlic_routing where it bundles several people’s messages together and onion routes that to make them harder to trace individually
freenet is just an distributed database anyone can publish to with “plausible deniability” that sorta self hosts everything pushed to it while making it hard to tell who pushed what originally.