a) An interface will automatically be "up" once connected - even without a corresponding entry in the configuration - right ?
Yes, that is correct.
b) If answer to a) is yes -> is there any security concern or impact ? What can an interface without a logical unit in Juniper do ? or rather what can an "up interface" without a corresponding entry in the configuration do ? Does it accept/transmit any form of traffic ?
or is there some sort of "default" configuration for an interface if it is not explicity configured/specified in the configuration
The physical interface will only have physical attributes such as MTU=1500, duplex mode = full, but no traffic forwarding enabled.
Interfaces have NO logical properties by default. That means no address or family (inet, inet6, ethernet-switching, and so on) enabling packet processing. All of these are configured under logical interfaces/units.
There is NO default configuration UNLESS you load the factory-defaults (like you saw in your switch) which adds family-ethernet-switching for example.
There is NO logical properties unless explicitly configured/specified in the configuration (either by you or with the factory default configuration).
c) Will the best practise = to make sure entries for all interfaces are configured in the configuration and set to disable ?
Some people do that for peace of mind, but I do not think that is necessary. Removing the factory default configuration (family ethernet-switching) would be enough.
HTH,