Many wrong answers here. Most devices have what’s called a power path. When a device is powered via a cable and the battery is full, the device battery is bypassed and the electronics powered via the charging port directly.
There are always losses in both charging and discharging a battery. Discharging the power bank first and the SD battery second avoids the charging loss from the SD battery.
You should avoid keeping either battery at sub 20% state-of-charge for long periods, if possible.