ssemenick Hi and welcome. It is a straight forward process that does not require any third party apps.
To export all your contacts as in a single .vcf
(vCard) file you need to login to your iCloud, go to your Contacts then tap the Actions button in the sidebar, choose Select All Contacts.
Tap the share button on the right and select export vCard.
Once you transfer that on your GrapheneOS device, open the Contacts app > Menu (top left) > Settings > Import > .vcf file
Many of our end users have migrated to GrapheneOS from iOS and had similar questions. Our forum has a search feature and it comes in handy for finding already answered FAQs related to iOS as well.