We recommend using mamba rather than conda (which is getting increasingly slow), see https://cylc.discourse.group/t/trouble-installing-cylc-8-1-0/584/2.
For installation on systems without internet access we use conda pack.
conda pack --dest-prefix=/path/to/cylc -o packed-env.tgz
The --dest-prefix option specifies the path to where cylc will be installed on the destination platform and avoids any need to “unpack” after untarring. https://conda.github.io/conda-pack/cli.html
We use conda pack for all our installations. On our build host we create 2 environments (using mamba), one containing the full installation and one containing the minimal environment needed on remote job platforms (cylc-flow-base + metomi-rose-base). We then pack all the required environments for the many platforms where we install Cylc.