How do I install Big Sur on a machine running Mavericks?

I just erased an MBP 2014 that had been running the latest Big Sur to factory settings in preparation for migrating from another MPB running Big Sur, following the instructions provided by Apple . But the process has installed Mavericks and provides no way to install Big Sur. How do I install Big Sur on a Mavericks machine? Can I just run Migration Assistant on the Big Sur (source) - Mavericks (destination) combo and end up with Big Sur on the destination? Is there some way to download a disk image to the Mavericks Mac that will work as an installer there?