In our case, the embryos were frozen, thawed, a few cells removed for pre-implantation genetic testing, frozen, thawed, and implanted again. Two embryos were implanted, but only one took hold. Then the one that took hold decided to become two people.
(We ended up getting singles twice, but we like to tell people they started as twins until we cryogenically froze one of them for two years)
(Also, for non-IVF, fraternal twins are also much more common, for different reasons.)
This is very country dependent and is actually changing quite rapidly.
I believe these days in the USA standard of care is single embryo transfer. It’s well known that twins are a much higher risk pregnancy:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5846681/
FWIW when my wife and I went through the IVF process our provider (RMA, who are basically a large US-wide franchise) informed us that multiple embryo transfer was not an option offered by their clinic (not that we would have chosen it, but point is it wasn’t presented as a choice).