Sync contacts between Outlook 2021 (PST-Folder) and outlook.live.com

Hello,
How can I please synchronize my contacts in both ways between outlook.live.com and my PST folder in Outlook 2021? Unfortunately the support of MS couldn't help with any question about synchronizing contacts or calendars and I didn't find any helpful entry to solve this basic issue.
Thank you for any help or advice & best regards,
Rico