How to understand, already installed SSL certificates are used where & how..?

You can see the certificates with their expiry info in the GUI

System  ››  Certificate Management : Traffic Certificate Management : SSL Certificate List

The GUI does not show where the certificates are being used, but you can extract that info with command line/tmsh:

 tmsh list ltm profile client-ssl recursive cert-key-chain

This gives you a list of certificate config items in the client-ssl profiles.

ltm profile client-ssl clientssl {
    cert-key-chain {
        default {
            app-service none
            cert default.crt
            chain none
            key default.key
            passphrase none
        }
    }
}