What exactly is an m3u list? How does it work?

Generally IPTV providers view 24/7 as a minor add-on and don't offer it as a stand alone product. Some providers will remove everything else from the playlist if the only thing you want is 24/7 channels.
You can also open a providers playlist in Windows Notepad or any text editor and delete everything except the 24/7 channels that you want to keep. Save the edited playlist as a file and use that in your preferred app.

yes you can edit an m3u list but u need to get a sub first. The one that used to work was on Kodi but there is also pluto tv too same as 24/7

I have subs with over 700 24/7 channels with more added all the time. Not bad for $8 $1 trial deducted if subbed.

Some providers make their own 24/7 channels off their servers