The military tried to physically prevent the National Assembly from meeting on a vote to disband the martial law. If they had been successful, the elected National Assembly would have no power. That's about as close to a military takeover as you can get without shots being fired.
From what I could understand from the news:
- the current parliament and the president are at war: the president vetoed any law the parliament passed because he doesn't like them. The parliament voted the budget and cut funding to whatever the president wished for.
- all political parties voted to have the martial law order revoked.
I don't know what the fallout of this will be, but the curent president of South Korea is toast. He went all in and lost big time.
Declaration of martial law isn't necessarily a coup attempt, in the same way that walking into the parliament building—hell, maybe even walking in armed—isn't necessarily a coup attempt.
On the other hand, declaration of martial law or walking armed into parliament might well be a coup attempt.
Korean law specifically prohibits presidential interference with the legislative branch even under martial law, a clause written in blood. The first thing Yoon did was try to lock down the legislature and arrest party leaders. This is a blatantly unconstitutional self coup attempt.