What kind of deployment are you planning?
In Foreman the default templates should be sufficient to deploy Ubuntu attached to upstream-mirrors via preseed using PXE in an environment where Foreman can manage DNS and DHCP
Depending on how advanced you want to have your deployments, you can also mirror your repositories using Katello. For the repository-management you will need a subscription-manager for Ubuntu. We provide the repositories for Ubuntu 20.04 as decribed here: Subscription Manager for Debian & Ubuntu on apt.atix.de
In this case you can deploy Ubuntu clients in an offline environment, where foreman/katello provides all needed repositories/packages.
If you plan to use the Ubuntu clients in a productive way and want also have features like errata, you might be interested in orcharhino which is based on foreman/katello but with focus on stability.