.onion URLs that I've seen do not open normal web browsing.
My question is can I open a typical website (like a .com or .net) and buy a .onion URL for it which can be opened through normal web browsing? in other words, can there be a .onion website that is accessible via the normal web?
If not, how is that? What is the technical difference between a .onion URL and .com?
No, you cannot buy a domain with a .onion TLD and make it available outside of the Tor network. The onion TLD is not in the DNS root zone, and the onion TLD is designated specifically for onion services.
The technical difference is that the routing and name resolution are completely different for onion services. Unlike typical websites, connecting to an onion service does not involve a DNS lookup and a TCP connection to the returned IP address. To connect to an onion service you must connect through Tor's SOCKS proxy, which will take an onion domain and then route your traffic using Tor's own protocols. This routing requires specialized software (the Tor client) and just a web browser by itself wouldn't know how to do it. You can read more about this in Tor's "How do Onion Services work?" document.
