> Can somebody please explain how was he able to do that without Congress approval?

Decades and decades of Congress generally refusing to do their job and also refusing to counter the ever-larger expansion of the Executive branch's assumed authority.

You might also note that the last time the US has declared war was WWII. [0] Vietnam? Korea? Afghanistan? Those weren't wars, they were "military actions", "military interventions", or "international police actions". [1]

[0] <https://www.senate.gov/about/powers-procedures/declarations-...>

[1] See the "The Korean War" section here: <https://www.congress.gov/crs-product/LSB11236>