That’s the old event building on Main Street that they use for Halloween mazes and stuff, right?
It looks familiar from when I went the back in 2018.
Yes, this building is called Plaza Grounds. It is one of the two in-park "black box" multi-purpose indoor spaces that HKDL has, along with the Pavillion (located between Adventureland and Grizzly Gulch). Although smaller in size, Plaza Grounds has always been used for more long-lasting events, with the Pavillion hosting very short-term ones.
A previous multi-purpose building occupied this space until 2015.
This specific structure was constructed in 2017, to house the 2017 Halloween maze. It was then transformed into the Royal Princess Garden during the castle's reconstruction period, acting as a place to temporary welcome Disney royalty. Finally, during the park's 15th anniversary, and to celebrate the Castle of Magical Dream's opening, it housed the exhibition "Building a Dream: The Magic Behind a Disney Castle", which closed at the end of the celebration.