Is there a current sense coil inside this ELCB?
Is there a current differential sense coil inside this ELCB (Earth
Leakage Circuit Breaker) just like GFCI or RCD?

No there isn't if the device is a true old-fashioned ELCB. There will be a coil but that coil will be sensing current flow through the earth connection like this: -

The ELCB relies on an intact earth connection to the downstream equipment and an intact earth connection back to the fuse board. It is the forerunner of the GFCI or RCD: -

The great thing about the GFCI or RCD is that it doesn't require an intact earth connection to be able to infer an earth current. That's where the differential transformer plays its role.
ELCBs are old-tech now and not as safe as GFCIs or RCDs.