If you are interested to see what you PC is sending out, you might want to take a look at Wireshark. Check if you can legally use it in your country and stay within the legal boundaries as you use it.
Some programs will ask you to be installed with administrator privileges. Be careful around them.
Oh, and don't trust the Cloud. You don't need a person wearing a ski mask and a hoodie to steal your stuff if you yourself upload your files to some server for convenience.
As for counter measures:
Encrypt your files. A stolen file that can't be decrypted might not harm you, while unencrypted files could. Consider using Bitlocker if you are on Windows (backup your files before you do, however!)
Delete what you don't need. Data can be used in rather interesting ways.
Consider using maintained Open Source software, if available for the task at hand. While you personally might lack the time or knowledge to check it, there is at least the chance that someone checked the sources. If it is actively maintained, this (or another) someone is working on it. See this page. Money quote:
[...] The rest is about trust as always on the internet.
On a different note, as you mentioned running Windows 7
As Windows 7 is slowly getting older, you might want to consider changing to another OS. While it will receive further updates until the January of 2023 (as stated here, text is in german) for paying customers, you can either upgrade to a newer Windows OS (if you need to run software that is Windows-only) or consider trying something like Linux Mint, which should make the transition between operating systems mostly painless.
EDIT: Regarding the question "How Windows 10 is different from Windows 7 in regard to my concern?":
Win7 is much closer to its end of life than Win10 in its lifecycle. I am not an expert on OS security, but looking at what happened with XP in the past, I'd presume that at some point in the future, support via update will run out and leave you hanging with an OS that is not safe to connect to the web whatsoever.
Metaphorically speaking, you might sail out upon some sea (symbolizing the web) with a boat (your PC running the OS). You may even poke holes into the side of the boat as long as they are far enough above the water line without too many issues, for easier access to the sea (your applications with web access). But if the boat start to rot/rust below the water line and you are out of maintenance gear/material (security updates), you might soon find yourself in a situation where the holes you poked into the side are the least of your problems.
I am, however, no expert on boats either, but I assume that a hole below the water line could be fatal.