But it seems not all my communities are linked. I can see them all in the "meta selector". When switching to some communities from the selector, I am logged in, others I need to re-login. I login to all communities using the same userid/password. It clearly knows "it's me" as my communities and points always show on meta selector for all sites, if I am logged into the present site. This makes no sense.
I came across this rather old question: "Account not linked with data.stackexchange.com" which suggests at the time (2011), not all communities are linked, "by (poor?) design" / intent.
Is this still the case? How do we know which are linked and which are not? Why would related communities not be linked and require logging in? Those I listed: Stack Overflow, Super User, Server Fault, and DevOps are all closely related, active users posting in the most appropriate community are likely to be members of most or all of those and switch regularly.
Finding yourself continuously having to login (if you've logged off) is most inconvenient.
Is there something wrong with my account? Do I have to synchronize my profile/settings? How? It seems to recognize my Email settings across all communities. This response to asking to separate accounts suggests accounts would auto-merge. Did that fail?
If nothing is wrong with my account, is there a roadmap to link at least clearly related communities? When? If not, why is this "by design"? Is there a reference document/page which shows which communities are linked?
Additional Info: Having logged out and cleared all the cookies (I also tried the following in private browsing mode), seems there is a distinct asymmetry going on. Logging into S/O also connects me to ServerFault, SuperUser, AskUbuntu, but not any of the other communities listed under Network Profile | Accounts. But if I log into StackExchange or says, DevOps, I find myself logged into all 20 sites.
In both scenarios, when I log out, I am warned, "Clicking
Log out will log you out of the following domains on this device: "askubuntu.com, mathoverflow.net, serverfault.com, stackapps.com, stackexchange.com, stackoverflow.com, superuser.com". I am not a member of 2 of those. It does however log me out of all sites I was logged into (either scenario).
I also confirmed this is the case both on my PC and Android device, both in Chrome and FF.
I reviewed @rene's answers and don't believe it explains this.