How to put title and page cover in Sigil?
 

For Kindle, I put page cover by using Calibre, which I also used to convert it from ePub to mobi.

For reading ePub files on Moon Reader, would I need to use Calibre as well to put book cover?

You mean in place of the Add Cover menu item? You may want to read Sigil's Users manual to see the major features.

I used "Add Cover" under Tools.
When I looked at it on Moon Reader, I noticed that the cover shows but the title doesn't show. I was expecting the title text to be on top of the cover. Is there no way to do that?

Most folk use the 'official' cover image. Those have the title and Author as part of the Image.
If I understand your wish, you want to use a plain image and have the (Title) Metadata superimposed. Nope

A Calibre user made Generate Cover plugin that allowed a (fairly) simple to be made: The Background image and up to 4 data fields placed. Not a whole lot of options on artistic styles. IMHO it is a nice option that offers more than the Generate cover in the MDE

Is this the plugin? https://www.mobileread.com/forums/sh...d.php?t=124219

Yes, that is the thread for the Generate Cover plugin.

By the way, does it only work on Windows?

Sheesh... Perhaps you should have asked that in the Generate Cover thread in the calibre forum's Plugins subforum.

I personally don't use Generate Cover but it installs and seems to work on Calibre running on SUSE Tumbleweed.

Calibre plugins generally work on Windows, Linux and MacOS.