My main question after reading some books on Staff Engineering, and Tech Lead roles is about how to be a Staff Engineering when you don´t work with an internal

My main question after reading some books on Staff Engineering, and Tech Lead roles is about how to be a Staff Engineering when you don´t work with an internal Engineering team in your company. This situation can be really common in non-tech companies that alwayes worked with partnership and hire development agencies for each specific tech project. How can someone seen as a the customer by the agency´s development team can have a chance to impact in their work. How can the Staff Engineer-Tech Lead be valuable for the agencies and influence them in how to proceed with the technical decisions? From my experience, I think the Staff Engineer will not shine, as he/she will struggle in being the technical lead of a Engineer team that are working in a different company.