A multilingual website is a single website that has different language versions on the same website. An example of this is to have a drop-down menu at the top of the website where you can select a language to read the website in.
Another example is to have a different language copy of the website as a subdomain (https://french.website.com) or a subpage (https://website.com/french).
Subdomain or Subpage?
There has always been a difference of opinion on this in the SEO world on this and sometimes depending on the latest algorithm changes the opinion changes. In the past, Google treated subdomains like separate websites now they don’t. The difference between the two in an SEO context is so small that it really doesn’t matter. My opinion? I used to use subdomains, but now I use subpages.
So should you use one website or several?
Typically it is better to use one website with several different languages because if one language does well on the website it will pass that success to the other language pages as well. However, by getting a country-specific domain name will boost your SEO efforts in that particular country a little more.
Here is a list of country-specific domain name extensions.
Information courtesy of Wikipedia.