Is possible to expand Modern UI current navigation by default despite of clicking arrows icons to expand it and collapsed it.
Currently this is not possible in SharePoint Online Modern experience by using OOTB functionalities.
You can read more about it on SharePoint Tech Community:
Can you set the Quick Launch Menu as to default as collapsed with Modern Pages?.
Also, I found one UserVoice suggestion on SharePoint UserVoice, you can vote on it:
Left menu/submenu expand and/or collapse default behavior setting.