I have located commerce-order-receipt.html.twig in commerce/order/templates/ and I have copied it to my theme directory, properly overriding it. My basic changes all work.
Now I am trying to figure out how to pass my own variable to it. I can't figured out which hook_preprocess function I need to use in order to do this. I have figured out how the suggestion and original variables were made in commerce_order.module but this has not helped me hook into it and add more variables.
Can someone point me in the right direction?