Getting error on Magento 2.3.4 at checkout that never existed before. What approaches should I take to fix?

Hello everyone. I work for a company that has a store built on Magento, the version we are currently on is 2.3.4. For a few weeks now the website has been producing an error in the console which comes into effect when a customer wants to check out the items in their cart. They can add the item, proceed to check out, enter their shipping information, and when they confirm their shipping information the following error appears when they are on the credit card information page:

Uncaught ReferenceError: Unable to process binding “if: function(){return (addressOptions.length > 1) }”
Message: addressOptions is not defined
at if (eval at createBindingsStringEvaluator (knockout.min.js:280), :3:55)
at ko.computed.disposeWhenNodeIsRemoved (knockout.min.js:376)
at Function.evaluateImmediate_CallReadThenEndDependencyDetection (knockout.min.js:208)
at Function.evaluateImmediate_CallReadWithDependencyDetection (knockout.min.js:204)
at Function.evaluateImmediate (knockout.min.js:202)
at Object.ko.computed.ko.dependentObservable (knockout.min.js:193)
at init (knockout.min.js:376)
at knockout.min.js:312
at Object.ignore (knockout.min.js:140)
at knockout.min.js:312

I’ve been given the task of remedying the problem. It should be said that the website was made by a freelance developer with whom the company no longer has ties. So it is fair to say that I have no idea what to do. My thoughts are to create a staging site, implement solutions from other people who faced the same issue, fix the problem, push changes into production and be the Hero the office needs, but doesn’t deserve. yay.

This brings me to my second batch of problems. I have SSHd into the server and I can not locate the application directory to copy so that I may create a staging site. Heck, I don’t even know If there is an application dir. The website is hosted on AWS, I have looked online for tutorials and documentation about Magento file structure but have returned defeated.

What do I want from this forum? Help. lots of help. I would like help with:

  1. Why is this error being triggered?
  2. Where can I locate the source code for the Magento site and how can I copy it to my local machine. I’ve found that SCP seems to be the most efficient method.
  3. After I copy where do I find the Models, Views, and Controller folders? I have zero experience with PHP but I am not intimidated to learn. (I have some experience with Angular and Django frameworks.)
  4. After I make the changes how do I push the changes to the live site? Can I use git? How?

I request detail. Lots and lots of detail with links and examples would be great.
Thank you! And I look forward to all the help