Single-Character non-ASCII Second-Level Domains
They known as Internationalized domain names (IDN), these domains are actually registered as their Punycode translations (which are more than a single character) for DNS purposes.
ICANN oversees a process for determining registration rules that involves wide-ranging stakeholder input and assorted Working Groups. In the case of .com domains, decisions are then implemented by Verisign, the contracted backend operator for the .com Registry. The result is a list of 96,957 codepoints allowed for IDN registrations. As mentioned above, some additional domains previously-registered are "grandfathered" and remain active. Many gTLDs also allow IDN domain registration.
These 96,957 distinct IDN characters eligible for registration in .com are the essential building-blocks of languages worldwide. A single letter domain does not provide the context found in a longer string or group of words. They may appear similar to one another, or look like English / Latin characters. They sometimes are used as pictorial symbols and memorable links.