The pre-installed Files app works for me:
Tap and hold the first file or folder in your list. Then, slide up or down to select multiple entries.
I prefer a list view with one entry in each row for a better overview.
After selecting at least one entry, the delete icon appears in the upper right corner. The three-dot menu right beside the delete icon offers further actions for your selection.
Or connect your phone with your PC to clean up your phone storage…