In your mac, you can open a web browser and access your contacts in murena.io cloud workspace.
Alternatively,you can install the nextcloud desktop application for macs that will synchronise files locally and offer you a link towards the contacts of your muena.io cloud account, that link points to the website of murena.io.
There is also a desktop app to synchronise passwords from the cloud workspace to your PC because integration of passwords is justified and useful (autofilling of forms with logins).
But regarding contact, there is no universal integration protocol across different OSs, therefore no Nextcloud integrated desktop app. You can simply copy the email address you need or download all contacts in .vcf format from murena.io and load them in your Apple environment.