The cool thing is this could take a million different forms, on both the creation and connecting sides.
The website builder could be generic and user-friendly, like Figma, or more niche and purpose-made, like a blogging platform.
Connecting sites is as easy as linking to them. So a site-building platform could make it easy for people to manage these links to friends' sites (e.g. a "blogroll"), and then publicly display those links or not.
I feel like you only really need other protocols when you want to notify people and/or have a private feed.
Of course blogrolls publicly show everyone you "follow" or "friend" on your site. But additional protocols and formats, like RSS for plain subscribing and #ActivityPub for notifying others, could enable a private home feed and everything around that.
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