Firstly, I have 0 experience with creating and deploying any kind of website. I want to enable Universal Links for my iOS app in order to use Plaid and I only need the domain for that, nothing else. For that I decided to use Digital Ocean and followed their basic tutorial for setting up a static website. From here I'm not sure where I am supposed to put the apple-app-site-association file so that it's available at the correct path.
So the first part of the location is will be whatever your document root directory is for your web server. That directory will typically be called something like html or public_html, but the exact path and name will depend on exactly which web server you are using and how it is configured. This digital ocean tutorial gives one example of what that directory path should be if you are using an Apache server on Ubuntu.
Then, according to the Plaid docs, the file should be located at
https://<my-fully-qualified-domain>/.well-known/apple-app-site-association
So within your document root, you'd have a directory in that directory called .well-known, and then the file itself would be called apple-app-site-association So if we were using the example path from the linked tutorial, you would put your apple-app-site-association in a directory whose path is the following:
/var/www/example.com/html/.well-known/