Links with //index.html and end and custom .html name files

sure, it depends on your permalink settings.
I think you set /%year%/%monthnum%/%day%/%postname%/ but you want /%year%/%monthnum%/%day%/%postname%.html ??

??

Is there a way to remove the //index.html? I have it on all pages and posts, and it simply doesn’t work like that. And I would prefer not having it called .html
So https://www.example.com/about/ instead of https://www.example.com/about.html

Thanks

if you want https://www.example.com/about/ instead of https://www.example.com/about.html than set inside wordpress permalink settings:
/%postname%/ instat /%postname%.html

The https://www.example.com/about//index.html normaly should be https://www.example.com/about/index.html . If you send me the debug data i can check your settings.

The index.html on local urls ending with a / is default on really-static, because some not well configurated servers have problems if there is only a / and not a /index.html.
alternative: https://really-static-support.php-welt.net/remove-index-html-extension-t14.html