Why does my Outlook mailbox automatically empty after retrieving mail from another Outlook mailbox or application?

I bought a new laptop with Windows 10 a year ago. I then installed the MS Office package including Outlook. Unfortunately, I had to find that after the inbox, the emails in the new Outlook disappear when I open the emails on another device (e.g. Apple 'Mails' app or Outlook on my old computer). The emails over 10 years are now saved on my old computer.

Which settings have to be made so that in the future this will also be the case for my new computer?

Could it be that your inbox is divided into read and unread? If you open the email in another program, it will no longer be displayed as new, but is not gone, but in a different tab.