It is very easy to make mistakes and break stuff, so please create backups for anything you wish to change. Many of the game's files are plain text and can be edited in any decent text editor. Many in the community use either Notepad++ or Sublime Text 3. There are 2 basic paths to changing game data: save file editing and script file editing.
Save files are found in a generalized application data folder with one of the following paths: (Replace USER_NAME with the value appropriate for your computer.)
Windows: C:/Users/USER_NAME/AppData/Roaming/Strive/saves
MacOSX: Users/USER_NAME/Library/Application Support/Strive/saves
Linux: ~/.local/share/Strive/saves
Save files store game data in the JSON format, so you will probably want to use an online editor or a plugin that supports editing that format. The save files have no extension, but if you add one the game will still be able to use them. While some values (like gold) can be easily changed, many of the values require knowing how the game is coded. Otherwise, you may think you changed something, but it will have no effect. Other fields are restricted to only a limited set of values and any other values will cause errors.
To understand how the game works and uses the data or to change how it works you may need to open the script files. Script files have the extension ".gd" and are written in GDScript, which is very similar to Python. Knowing where to find specific code or data for Strive takes a bit of experience with the code, so the fastest way to find something is often to ask someone who knows it better. There is a Strive Discord(https://itch.io/t/284398/discord), where modders can often be found for answers. That said, much of the code is rather simple so if you can find the right parts it is fairly easy to understand. Note, there are some sections of code that are no longer used, so not everything you find will be helpful.