There is a widget area named “Copyright Area”.
This can be found at ‘Appearance’ > ‘Widgets’, or ‘Appearance’ > ‘Customize’ > ‘Widgets’.
You can add any widget to that widget area and it will replace the footer ‘Powered by’.
So for example if you want to add your own text, you can add a “Text” widget and write your own content in the text widget.
If you just want to remove the theme text and leave an empty space you can add a text widget and leave it empty.