how to place the contents of emails below the email list

Go to menu View > Layout and check ✓ Message pane... or simply hit the F8 key.

that was already check yet I still have to double click the email to open a new tab that shows the email content. in the image you can see the bottom of my email list, with a dozen above it.

You forgot to add your screen shot! Hit the "Browse..." button in the lower left corner and navigate to the image file you want to add.

Maybe you moved the horizontal message pane separator (message pane splitter) too far to the bottom of the Thunderbird window? (see image)

I did but it was small, hard to see. Here is a better one. Does this help.

No, the new screen shot doesn't help either. I can't see what is below your thread pane. When you restart Thunderbird in 'troubleshoot mode" do you see the message pane? Help > Troubleshoot Mode ..... https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

there is nothing below the pane. Here is another screen shot of the top of the app. showing the tabs. As I said, I have to double click an email to see the contents in another tab.

I repeat: if you use the Classic view as shown in my new screen shot and if you can't open an email with a simple click but have to double-click that it opens to be read in a tab, that means that your message pane is hidden. To activate (display) the message pane go to View > Layout > and choose "Message Pane" .... or simply hit the F8 key. Thereafter your main window should look somewhat like in my picture.