Where do my Bank Transfer instructions appear for the customer?

Your Instructions text is shown on the order confirmation page, right under your bank account details, after the customer places a Bank Transfer order.

To set it:

1. Go to Settings → Store Settings → Payment Gateway and click Configure on Bank Transfer.
2. Type your note in Instructions, for example "Use your order number as the transfer reference."
3. Click Save Changes.

At checkout itself the customer only sees that the bank details will appear after ordering. UPI has its own Instructions box, shown with the QR code.