Tor (acronym for The Onion Router) is a very highly encrypted proxy that makes use of several encrypted layers. When you connect through the Tor network, your data is encrypted several times through different nodes and it becomes impossible for anyone to decipher it. The Tor network also assigns you a different IP address and thus hides your real IP address effectively. Tor also allows people to host their own sites that have .onion TLDs. Since the servers hosting these .onion sites are untraceable and their URLs are also generally not found on mainstream search engines, these sites are also called darknet sites or darkweb.
Anyway, if you have a .onion site of interest and you do not want to use the Tor proxy bundle (Tor comes in form of a software bundle), then there is another way of accessing these .onion sites – through the use of another service called Tor2Web. This Tor2Web service makes .onion sites accessible without having to use any proxy. But this comes at the cost of revealing your online identity, since you won’t be using any proxy. Your online identity will be visible to both the Tor2Web and the .onion site that you intend to visit.
For using the Tor2Web online service, that acts as a small bridge between the .onion sites and the regular internet users who are not interested in hiding their online identity, all you have to do is append .to, .cab, .city or .direct to the .onion domain name. For example, if you want to visit http://3g2upl4pq6kufc4m.onion, then you can append .to after .onion and it becomes http://3g2upl4pq6kufc4m.onion.to which can be accessed from any web browser without having to use Tor (by the way, this .onion site opens up the official DuckDuckGo search engine site).
Conclusion: It is possible to visit the .onion sites without having to use the Tor proxy. But this will expose your online identity and defeats the whole purpose of the Tor proxy that intends to keep the users anonymous.
You can visit the Tor2Web website for more information at https://tor2web.org/.