This documentation is not using the current rendering mechanism and will be deleted by December 31st, 2020. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.

Changing the company field (in the billing address in the registration form) from input field to textarea

This can easily be done via TS setup:

plugin.tx_seminars_pi1.form.registration.step1.elements {
  company = renderlet:TEXTAREA
  company.custom = rows="3" cols="20"