If you want to use the KDE/Plasma KDE Online acounts system settings you have to install a bunch of small plugins.
To get google account button visible install both of kaccounts-integration kaccounts-providers
To get other on-line account buttons mostly for chat and messaging such as (AIM, ICQ,Jabber, KDE Talk, IBM Sometime, Gadu-Gadu, Office365/Lync, Telegram, Yahoo Messenger and Skype) to work install telepathy-kde-meta (which installs the rest of the needed things for this type of account.
Webdav Accounts: You shouldn’t need to install anything extra for this to work, nor are there any plugins for system settings online accounts page yet.
(You can try this directly in Dolphin by using the Network Places feature. For instance, to connect a free Yandex webdav drive you use your email name, and webdav.yandex.com, and select use encryption, and then enter your password when asked. )
Just because you have successfully set up an account in the account page doesn’t mean it does anything yet. This is just a place holder for logins and passwords, etc. It doesn’t create any connections.
The info stored in the online accounts page will be used when you use other KDE programs, especially those in the KDE PIM Akonadi group of applications when configuring new accounts. I can’t vouch for the usability of this repository of accounts, but it is supposed to work. Its just that Manjaro sort of forgets to add all the pieces and parts when it installs KDE.