Why many unrequested 6-digit SMS login codes sent to my Nougat Phone?
The most likely source of your multiple two-factor confirmation messages is something making multiple attempts to log into Google or Google+.  This might be malfunctioning apps on your device, or someone else (accidentally or intentionally) using your Google ID from another device.  You should contact Google, who will be able to confirm the device that was used for the login attempts; if it's your phone, you'll want to look at what app might have a wrong stored password, or that would have been closed and reopening (hence relogging) at those times.

If it's not your phone, you'll want to change your Google password immediately: these two-factor codes aren't generated until after someone has entered your valid password during a login attempt.

You note above that:


  Signal app is also sending me verification codes at the rate 1/day, although I uninstalled it. This fact suggests me a software issue. I cleaned the cache, but not sure if enough.


This suggests that your OS isn't fully under your control.  Whether this is due to malware or just a glitch of some sort, things you've uninstalled aren't going away as they should.  I'd suggest backing up any important data (photos, music, text conversations, etc.) and performing a factory reset on your device.

If that fails to clear up the problem, you should take the device to one of your carrier's shops (carrier owned/operated, not franchised -- there's a big difference in the level and type of support available) for further evaluation: your device might have been cloned, a situation that can lead to shocking bills and a lot of headache trying to clear up the charges.