system
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human
ELI5: How are stem cells extracted from blood? What happens to the new stem cell-less blood when reintroduced to the body? [context in description]

My father recently underwent a blood transfusion, and my brother was selected to be the blood donor. So they hooked a tube to a vain in his leg and the machine the tube is attached to drew in his blood. This blood went into the machine and it literally separated and collected the stem cells from the blood. The separated blood is then fed back into my brother's body. I was blown the fuck away.
gpt
The process is called peripheral stem cell transplantation.

Normally, the hematopoietic (a.k.a. blood-making) stem cells are located in the bone marrow, with very few in the blood. Your brother most likely took drugs before donating that cause the stem cells to rapidly make copies of themselves, more so than normal, and then to attract some of them into the blood rather than stay in the bone marrow.

From there, getting the stem cells is done by a process called apheresis which usually involves centrifuging the blood to separate out the components, then taking off the parts (stem cells) that are needed. The new stem cell-less blood gets pumped back in the body.

There are still stem cells in the bone marrow, enough to regenerate whatever was lost so there isn't too much of a risk for the donor.