Right click on the number of notifications that is next to the clock date and time on the task bar (extreme lower right corner). Select notifications settings. You can turn off notifications on the settings window that pops up.
Some notifications are program specific so the won't appear as on/off slider options in the normal Windows > Settings > etc options.
Click (normal) on the notifications number. The popup menu will list where the notifications apply. Mouse-over (NO click) each notification will reveal menu options/symbols, one of which will be an elipsis (3 vertical dots). Click on that for the next popup menu, one of which will be (if possible/available) to turn off all notifications for this. Has to be done for each one.
As for the security popups try opening Windows security in the all apps list and select the settings icon bottom left. You should find the notification settings there.
All those listed above are, indeed, options within the Settings menu. Open the Start menu, and without clicking anywhere, start typing a key word to find the setting - e.g. privacy, leads to Privacy settings. Click on that and choose your options.