The three buttons

Open in Explorer button

This button will open the File Explorer at the path of the selected entry's file and select the file - intended to allow quickly interacting with the file without having to find it manually. If the file no longer exists it does nothing.

Check Online button

This button will get the latest chapter from the MangaDex API and compare it to the Last chapter set in the NumericUpDown control. This requires the link to be set correctly. Should there be (a) new chapter(s), the Download New Chapters button is enabled, leading to the downloader in its second mode.

Delete Entry button

This button allows the user to delete the selected manga from the list, offering the option to delete the file as well.
The program deletes the file, it does not move it to the Recycling Bin!