How can I retrieve off-chain stored data/files? For example, if I upload a file during a function call, how will I be able to retrieve it?

Utilize the transaction bundle file or transaction bundle file by name endpoints in your API to retrieve off-chain files.

You can utilize the transaction bundle file or transaction bundle file by name endpoints in your API to retrieve the files. You can also use the methods provided in the SDKs, such as https://simbachain.github.io/libsimba-js/class/src/simba/simba.js~Simbachain.html#instance-method-getFileFromBundleByNameForTransaction