I read https://maas.cloud.cbh.kth.se/MAAS/docs/ui/try-out-the-maas-cli.html
from the text it seems it is sufficient to call
$ maas admin boot-source-selections create 1 os=“centos” release=“8” arches=“amd64” subarches="" labels=""
$ maas admin boot-resources import
and the deployed system becomes Centos. It does not - I get Ubuntu 20 deployed. I checked importing is done yet id in fact is 11. How should I install Centos?