Could you give more details about what you mean by "homemade internet"? The first thing I thought of was just a LAN or Intranet which I see many people have already suggested but I feel like that may not be what you're actually talking about
This could have been a really interesting question if OP hadn't been so vague. As is, there's too many interpretations to answer. Do they mean the physical connections? The protocols and services like IP, DNS and BGP? The world wide web, with its sites, links and search engines?
Does OP consider the Dark Web its own internet? Or a large corporate network its own internet? What about self-hosting a huge number of services in your own home?