This is a Matrix widget — it searches your indexed message history and only works embedded in a Matrix client such as Element Web, where it can talk to your matrix-search bot through the Widget API.
To use it, open your search room in Element and add the widget:
/addwidget https://matrix-search.marcade.datanauten.de/?theme=$org.matrix.msc2873.client_theme&userId=$matrix_user_id
Then pin the widget in the room and grant it the requested permissions. Search runs over room events answered by your own matrix-search bot — nothing is sent to this server.