If you want to self-host the actual free open-source software, you can totally do that. You just shouldn't go looking at the product page of the enterprise offering for it (:

It looks like ESS is mostly managed deployment of otherwise opensourced software like Synapse, Element Web, Element Call, and friends. And I guess they have some backoffice with a glossy web UI for account/user management.

https://github.com/spantaleev/matrix-docker-ansible-deploy is a good starting point!