The flags are in some of the fields in the national mall. It's a series of long fields between the Capitol and Washington monument with significant buildings around them. It's a decent distance. From the vantage point in this video you can not see the extent of the fields across the mall (where the flags are).

A couple pics for reference: https://www.dccirculator.com/explore/where-we-go/national-mall/ https://www.ksat.com/features/2021/01/19/thousands-of-flags-flooding-the-national-mall-is-stunning-and-the-reason-for-them-is-moving/