Hi @SGH ,
Per my knowledge, for now there is no OOTB option to set the left navigation expanded by default. Microsoft set the left navigation collapsed by design in SharePoint 2019 modern experience. If you want to these link expanded by default, you could try to use "New Heading" instead of "New Navigation Link" as a workaround.
Can you set the Quick Launch Menu as to default as collapsed with Modern Pages?
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