“Synthetic” in this case doesn’t mean that it’s made using an alternative chemical process - it just means they’re not made in bone marrow.

This process filters out stem cells from a blood donation and prevents them from becoming red blood cells long enough to multiply them significantly.

Yes. Some of the proteins in the cells would be different to the host cells, due to the exogenous stem cell source. This might be detectable with mass spectrometry (protein sequencing is otherwise difficult and laborious). There might also be different post translational modifications on the lab cells.