You file a document (like an Answer or Motion) with the court when you bring a document to the court clerk or file the document online through the Texas online filing system, found at https://efile.txcourts.gov/ofsweb.
You serve a document by providing a copy of a document to the State and any other parties to the lawsuit.
Documents like Pleadings (including Petitions, Answers, Motions, and Notices) must be filed with the court where the case is taking place AND served on the State entity that filed the original forfeiture suit (like the District Attorney, if that is who filed the original Petition), plus served on any other party to the lawsuit.