.htaccess [L,QSA] flag

#Before: https://example.com/index.php?user=robert

RewriteEngine On
RewriteRule ^user/([^/]+)?$ index.php?user=$1 [L,QSA]
    
#After: https://example.com/user/robert?query=value

#You can also use other query too.

#Like: robert?query=value&query2=value2&query3=value3