Thank you, DaveM121, for your advice.
After all, it seems that I am using Office 2016, though its layout has changed very much.
Upon installing Windows 11, Microsoft Copilot was installed without my knowledge. I guess that’s why Microsoft 365 de-de, en-us, ja-jp appeared in the list of installed Apps, which were not there before.
If possible, I would like to reinstall Office 2016 on my PC, as you suggested, but it seems not possible. I see none of Microsoft Office Home & Student 2016, Excel, Word or PowerPoint in the list of installed Apps, so I cannot uninstall them. I need to uninstall them first before the reinstallation, to download Microsoft Office Home & Student 2016 using my license from Microsoft. My account on Microsoft does not show the possibility to download Office 2016 using my license. (Is it because the license is in use?)
It's very frustrating that Microsoft changed Office settings without my permission. I uninstalled Microsoft Copilot, but it didn’t solve the problem that I cannot see some old PPTX files.