Shopify runs on Ruby
Yes mostly, but the api is rest and has libs in php, store templates are html,js,css&liquid
Thank You Page
Assume you mean shopifys thank you page after order completion.
Have you contacted https://apps.shopify.com/revenue-conduit-for-infusionsoft for your use case?
Or other apps that integrate infusionsoft?
There is also the shopify app 'querstring' for dealing with url parameters but ,by design, shopify themes (liquid) are safety scissors so there's not form processing like in php or ruby to make rest requests.
You'll probably want https://docs.shopify.com/api/uiintegrations/application-proxies
Or build it in ajax or a full app:
http://ug.infusionsoft.com/article/AA-00878/0/Track-referral-activity-via-the-API.html
You should hit the shopify api's & tech forum for advice on getting details together. Though most of the info on the forums for stuff like this is about mailchimp.