An 'internal' link is a 'hyperlink' to a page on the same website. They can be absolute or relative. An absolute link contains the full web address of the page being linked to. A relative link is relative to where you already are. With superMicro CMS relative to where you already are is the same place, eg:
https://example.com/ and not
i.e. you are linking 'internally' from a page on https://example.com/ to another page on https://example.com/ which means that links are simple.
A relative link on Apache web server:
<a href="./my-page">link text</a>
A relative link on Microsoft-IIS (Windows) or NGINX:
<a href="./my-page.php">link text</a>
The only difference is the file extension .php on the end with a Windows or NGINX web server. Most likely your website will be on Apache Web Server so the extension is not required. Make sure you know whether your website is on Apache or not so internal links are written correctly.
To deal with situations where a superMicro CMS website is moved from Apache to Windows and all your internal links do not have the .php file extension, the CMS automatically adds it for you, so you don't need to edit them all. It matters less moving the other way from Windows to Apache because the CMS rewrites all its web addresses to remove the suffix.
Note 1: this applies only to internal links written 'relatively' without a full URL and linking to a page within the CMS. Anything else remains untouched.
Note 2: from 14th September 2020 this feature can be disabled in Admin / setup / .php it YES/NO.
Regarding absolute vs relative internal links, either will work but it's irrelevant which you use because the CMS automatically converts relative links to absolute. Just about every internal link with superMicro CMS is absolute, including internal links to images, video and stylesheets. Amongst other things it will help protect the website from automated content scrapers – at least make it more difficult for them to make much use of it.
Just a normal external 'absolute' link:
<a href="https://www.example.com/">link text</a>