Excel is open automatically upon windows start
Solution 1:
It turns out that this may be caused by a so-called feature of Microsoft.
Short answer: Windows Settings->Accounts->Sign-In Options->Privacy->Off
Longer answer: https://answers.microsoft.com/en-us/msoffice/forum/all/microsoft-word-and-excel-2016-automatically-opens/8d5869df-0212-4f04-9fac-c7e99256a005