How can I remotely, outside my network, access files on my Windows computer from my iphone over the internet, using mobile data?
Actually what you are trying to do has nothing to do with Apple products, per-se. But is a fairly common task and something that I used to do when I hosted my own website off an old Mac attached to my home network.

This applies to any networked device that needs to access another networked device from outside one of the networks, like this.

iPhone --> Internet --> Your home network --> Your PC

The trick here is that you have to configure your home router to forward specific connection requests from outside to specified devices inside your network. This is commonly known as port forwarding and the linked article on Wikipedia will give you some info on the theory of how it works.

Most home routers that cable companies provide allow you to set up port forwarding. And every single one of them is different. So you will have to consult your ISP (they may have articles on how to do this available online) on how to do it with your specific make and model of router.

You will then need to set up your Windows PC with a static IP address and point the ports you are forwarding to that PC.

You will need to search out what ports the Apple Files.app uses and forward those ports on your router to the IP address of your PC.

Not to mention setting up a DNS name for your system as your router's internet accessible IP address will change from time to time. There are free services that can do this too.

I have never done what you are trying to do, this is just a basic overview of the process.

One thing you will have to take into consideration is that you are opening up your PC to the entire internet. So at a minimum you will need a good strong password on that file share. And you will also need to hope that some bored hacker doesn't find your system and try and break in and destroy your PC from somewhere in Romania.

Honestly, your best bet (and easiest route) is to do what Nimesh said in his first comment, use Dropbox, Google Drive, or any other similar service to do this. Introductory tiers of these services are often free and a LOT easier than setting up your own Internet accessible file share on your home PC.

Also I believe there are home NAS units that come with software that will do this. Also easier than what you are planning.