Opening .xlsx file in Numbers App in macOS 11.5.1
How to open .xlsx fine in Numbers App
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Launch Numbers, in the menu go to File > Open, and choose the .xlsx file.
That's it! Numbers will then try to convert it to a Numbers file.
The results may not be perfect. The apps are very different, and some formatting may be lost and not all formulas may work. If you are collaborating with Excel users and need to send files back and forth then you will be better off using Excel for the Mac, or possible LibreOffice or another of the Microsoft Office clones.
But for the occasional import of Excel documents Numbers does a pretty good job, perhaps good enough to meet your needs.
SG