Save as xlsx & pptx files by default with Calc & Impress [closed]

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Yes. Done from Tool -> Options -> Load/Save -> General panel.

In the Default File Format and ODF Settings section there is a drop list for "Document type" and below that a drop list for "Always save as".

Select Spreadsheet for the document type and Excel 2007-2019 (or an Office Open XML Spreadsheet if standards concerned). The pairing will be recorded to the LibreOffice user profile.

However--these formats are not "native", they are export filters and on roundtrip use an import filter to bring them back into LibreOffice. Integration of ODF standard(s) formatting is much tighter. Unless you have a real reason to work in Microsoft XML you'd have more reliable document handling in "native" ODF.