What is the difference between htm and html? What is the difference between HTM and HTML_HTML/Xhtml_Web page production

In the process of learning web design, I did not delve into many of the problems I encountered, such as the relationship between html and htm. I have sorted it out today, hoping it will be helpful to you.
The most common extension for files containing HTML content is .html, but older operating systems like DOS limit extensions to a maximum of 3 characters, so the .htm extension is also used. Although it is less commonly used now, the .htm extension is still generally supported.
Both are extensions of static web page files. The extensions can be replaced with each other without causing errors (this refers to opening, but for a link, if it points to an html file, and that htm file is changed to html file, then the link cannot be found)
HTML loads faster than htm
htm comes from the era of DOS three-character file extensions
　html supports multi-character extensions for Windows era