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?
If you are executing MAAS CLI machine deploy then it is not enough for MAAS to decide which image you are deploying (without distro series parameter provided, it will take default one).
Can you please try the following procedure to see if this fixes it: