Hello!
The KB4489622 is the CU 23 for Exchange Server 2013.
The KB5019758 is a Security udpate. ( If Microsoft detects vulnerabilites in Exchange until the next CU is available, a security update for Exchange is launched for the installation ).
About the Schema, when you install your first Exchange server you already need to prepare the Schema for it. After that, if you don't miss any CU, you don't need to worry about that, as the CU takes care for you. Anyway, at any CU installation documentation you'll see which are the changes and te requisites.
If you plan the CU updates for your environment correctly, you should not face any problem. :) ( Keeping an eye to the possible vulnerabilities detected and corresponding security updates ).
Hope this clarifies your doubts. Have a good day!
https://support.microsoft.com/en-us/topic/description-of-the-security-update-for-microsoft-exchange-...https://www.microsoft.com/en-us/download/details.aspx?id=58392
