groups in nextcloud

Nextcloud is a very interesting, open source online platform. With /e/ you have a cloud of 5 free Gb with calendar, contacts and tasks shareable between PC and smartphone.
1) First of all you should export your data (calendar, tasks, contacts) from Google; a problem arises with contacts because Google export your contacts in a vcf file without groups.
2) So you have to import your contacts from vcf google file in nextcloud and re-create groups, which this time could be saved in a vcf file with groups (as categories).
3) To see your contacts and groups you have to use DAVx⁵ and in account settings - > cardDAV -> group organization you have to choose as in the following image:

not [ ] groups are separated vcards
but [x] groups are per-contact categories
4) This way you will be able to see your contacts and groups in your phone app (or in whatever other app).