The non-scammy way this happens is senior engineers are part of the interviews and requirements gathering. They do the design and estimation. They develop task and proof of concept code for junior engineers.
During the work the senior engineer almost never 100% on a single project. They are on three different projects in different stages: design/early development on one that just started, resource and mentor on a second that's been going a while, writing quote for a third, and initial sales contacts for multiple other.
Based on availability it might not be the same senior person at any step of the process.
It's hard to impossible to a give you the same person who was part of the initial contact because by the time you get teh PO approved they are already hip deep in something else.