Windows intrinsically uses the UTF-16 encoding. The “default encodings”, or “default codepages” in Microsoft terminology, are just legacy encoding that are used by many old programs. E.g., in Notepad, the default encoding when saving a file is “ANSI”, which (rather misleadingly) denotes the default codepage (but Notepad internally works with UTF-16).
The system locale determines the default codepage. The page Windows Locale Codes - Sortable list shows the mapping and is probably reliable. (I was unable to find a Microsoft page on this.)
In particular, web browsers tend to use the default codepage so determined as the encoding of a web page, if encoding information is not available. In reality, it’s more complicated. In HTML5 LC, section Determining the character encoding describes (and prescribes) the process, and it also contains a mapping table that largely reflects the Windows mappings from locales to codepages.