I'd like to be able to create and edit google sheets from jira automatically (i.e on creation of a subtask/issue)
These google sheets will not contain any jira fields - they will contain other data.
I'd also like to display the link of said sheet in the jira issue.
How can I do this?
It may be possible to create a sheet using existing apps on the marketplace but I am quite sure you can also do that using Google APIs. Take a look at this page, now you will have to write a script whether within Jira using an app like ScriptRunner or outside Jira and trigger this sheet creation using a webhook. These are some possibilities, I hope it helps.