As described in the article I linked above, if you want bootstrapcards.html to be your index file, you can either rename it to index.html, or set a DirectoryIndex rule:
If you just want a directory listing of your files, this is also covered:
You can create a file with the name .htaccess in the directory and add the line DirectoryIndex yourFile.html index.php index.html index.htm, where yourFile.html is the name of the file that should be loaded.
No worries.
If you want your boostrapcards.html file to be displayed when someone visits your site, without changing its name to index.html, you can create a .htaccess file in your htdocs folder with this content:
This tells the server to look for a file called “bootstrapcards.html” first; if it doesn’t find one, it will then look for an “index.php” file, then “index.html” and finally “index.htm”, and failing all those, it will redirect you to the 403 page.