Is there a convention for how to order UTXOs in a transaction with multiple inputs? I am not sure it matters to produce a correctly formatted transaction, but I am playing around with a wallet that does not keep its own private keys, but produces the raw txs to sign and want to give the user the ability to verify the details of the raw txs with a third party or some control libraries. Assume, Bitcoin Core sets the standard, but I am unfortunately C illiterate. Thanks, Kyle
The convention is to have the outputs in random order. This is specifically done in an effort to conceal which output is the change output. The change is by default sent to a new address that is owned by the sender, and as such has no history associated with it.

The majority of bitcoin's code is actually c++. Here is a link to the relevant transaction creation code, maybe there are a few comments that could point you in the right direction.