Your questions is very generic, it might be flagged by some moderator later. Anyway... for SharePoint Online (not SharePoint 365), you will need to write your solution using the SharePoint Framework (SPFx), to achieve this you will need to familiarize yourself with TypeScript, NPM, GULP, Yeoman Generator, React JS, and a to some degree of JavaScript.
The other possible way is to familiarize yourself with the Office 365 architecture, this options might be limited to fully achieve your goals but might be the best option if you are not a developer.
Basically you will need to create a "workflow", you can create using Flow (when a list item is submitted), inside the Flow you will read the fields submitted and send for user manager's approval (reading the property from user profile), and once the "workflow" is approved, you can capture the data, and send to Excel (you can even specify to which Excel flavor you want: Excel Online - Business, Excel Online - OneDrive, or simply Excel).
Once your Flow adds the information your target (Excel file), you add a new step in your Flow to use the "Adobe Sign" to use the create a certificate - you can use this step to get a document from an signed agreement or any other you find suitable there
You have the hard way by coding through SPFx or making your way through Flow (the easy way), it is now up to you, go with Flow, you will achieve this in probably a couple of hours.
