Like a lot of you here, Mama Donya saved me.
The Flag Smashers gather in a large room of an old factory for Donya Madani’s funeral. Sam and the others enter into a lower level and eventually chase Karli out of an upper level past steam pistons and other old equipment.
Apparently this location is an old wastewater treatment plant (Stará čistírna odpadních) circa the turn of the 20th Century. There are various levels where the characters were able to run up and down stairs, plus the big open room for the funeral, which is on the 2nd level on the North side. Thanks to the blog post at World Heritage Site, for having an image of the steam engines featured prominently at the end of this sequence (6th image below), making it easier to identify.
(The Falcon and The Winter Soldier S01E04, 2021)