Bow et al wrote:
"We know normal conductor current form a closed loop, is this also true
for displacement current?"

AC circuits can be completed by capacitance. Countless coupling
capacitors demonstrate a-c flow on and through conducting parts of a
circuit. The same current flows via displacement through the dielectric
which inhibits charge conduction.

AC current is the same through a series path but moving charges that
produce electric force may go only back and forth in very short paths.

Conduction causes displacement and vice versa.

Best regards, Richard Harrison, KB5WZI