Welcome, @fernandomaposse.
ODK Build was a form builder that the ODK project produced. We no longer support it and generally recommend building forms using XLSForm. See Build permanently shut down on Feb 1st, 2024 and the ODK getting started guide.
The ODK server is called ODK Central. You can self-host it for free or pay for fully-managed install on ODK Cloud. There are many other tools and projects that are compatible with ODK Collect and some do offer free accounts.