Your checkout complete page https://domainname.com/checkout_complete
Note: multilingual sites will have the locale in their slug for translated pages, e.g. https://domainname.com/de/checkout_complete
The details here are not editable by you on the EN page. Your company name & the booking ID are automatically inserted into this template. All other information is "boilerplate" and cannot be altered. If you have a multilingual website however, on the non EN pages you can translate this content into whatever you want.
search for "shortly you" or "what happens next" etc * provide your translation.
*** CLICK HERE Translations - Multi Lingual Site *** - Full instructions on translations
If you want some extra control over this page, you could add in a block (content / image / HTML etc) by editing the page & inserting blocks in the standard way