Could be anything from cosplay to historical re-enactment.
Blokes do make costumes, you know... If only because buying tailor-made one-off stuff is prohibitively expensive...
As for the uniform.. Like the sailor outfits of japanese fame, this one looks military inspired as well. Basically faux cavalry chest armor.
In this case interlining ( possibly felt ) and starch are your friends. Not unlike certain styles of 15th/16thC doublets.
There may also be hidden buttons or hooks involved.
Like those US civil war outfits, those panels regularly got hardened leather or metal plates sown in/on. For obvious reasons.
The shirt tassels in the front are the remnants of the points where you mounted your leg armor ( or "socks"..) on.
And guess how those impossibly high boots stayed up...
So yes... they go
over the pants.
And yes boys and girls... the garter belt is originally
not a female item of underwear. It was a functional and necessary piece of a set of armour.
Then again... ladies nowadays use the things for their own kind of Warfare...