The short answer
We work through the screen, battery, electronics, antennas and sensors together. The goal is a product that can be worn and used comfortably, with enough space for assembly and reliable operation.
In more detail
Board layout and mechanical structure need to develop together. Dense circuit boards or flexible connections can help, but they also affect cost, repairability and manufacturing checks.
We compare size, weight, battery life, heat and signal performance before fixing the package. A different wearing position may change the antenna and sensor design even when the device looks similar.