
Explanation

New-ExchangeCertificate: Create the certificate renewal request.
Import-ExchangeCertificate: Install the new certificate.
Enable-ExchangeCertificate: Assign the new certificate to Exchange services.
Reference:
https://docs.microsoft.com/en-us/powershell/module/exchange/new-exchangecertificate?view=exchange-ps
https://docs.microsoft.com/en-us/exchange/architecture/client-access/import-certificates?view=exchserver-2019
https://docs.microsoft.com/en-us/exchange/architecture/client-access/assign-certificates-to-services?view=exchse