Typically with a time machine. .unity3d files are no longer produced by the engine. Nor do they run on any modern browser.
The current standard is WebGL. Getting WebGL up and running on a website is trivially easy. You simply dump all of the files that Unity outputs from a WebGL build onto your web server.
I’m unsure if Wix supports uploading files directly to the web server. You might need to host somewhere else and embed them.
But that solution opens up yet another can of worms as the basic plan only provides you with 2GB bandwidth. Getting a plan that removes the bandwidth limitation and allows your site to be useful is $17/mo. You’ve now surpassed the cost of just about every shared hosting provider out there while having worse features (most of them provide effectively unlimited space in addition to effectively unlimited bandwidth).
Wix is barely acceptable for a free website. Once you need a paid website it’s one of the worst options available. If you don’t have any skills in making your own website you’re far better off with a company like Square Space which has more features for about the same price.