So, the difference between dual standby and dual active dual SIM Android phones are:
No difference when the phone is in standby mode.
When you are using one of the SIM cards for voice calling (or sending SMS) through 2G network, the other SIM card will be disabled in dual standby phones.
4G+2G dual SIM phones can connect only one SIM card to 4G (or 3G) network. The other SIM card can only connect to 2G. Usually, the SIM card slot does NOT matter. Most dual SIM Android phones allow you to specify and change them.