Yandex is a Russian search engine which also provides personal services like email, address books and calendars.
If you have a @yandex.ru account, choose "Login with email address" and enter your Yandex email address and your password.
If you're using Yandex with a custom domain, choose "Login with URL/user name":
Base URL: https://yandex.ru or https://calendar.yandex.ru (for CalDAV)
User name: your Yandex email address (firstname.lastname@example.org)
Password: your generated app password (when app passwords are active for your account) or your Yandex password
Contact group method: groups are per-contact categories
Only the group "Phone contacts" in the Yandex address book will be used for CardDAV (i.e. be synchronized with DAVx⁵)! You can assign contacts to other groups and "Phone contacts".