Use Google Apps Script, Google Slides and Google Sheets to quickly automate creating a Certificate of Attendance and send it to attendees.
Nice example using Google Apps Script to generate and share .pdf documents via email. The source has all the code you need to get going on this.
Source: Google Apps Script: Automated PDF Certificated Creator That Is Emailed to the Attendee – Yagisanatode
Member of Google Developers Experts Program for Google Workspace (Google Apps Script) and interested in supporting Google Workspace Devs.