This is more like giving fish rather than teaching to fish BUT I had a hard time implementing this myself for my idle game so I wanted to share.
Download the sample project first:
What this does is basically getting the number's first X digits depending on the number,
Then selecting the unit from the array based on how many zeroes does the number have.
If you want to implement this method in your own game, simply create a similar function and call that throughout your project.
Note that this method by no means lifts your variables' limits, they will still cause problems when they are over C3 bounds.
In order to implement the function in your game
Step1 : Create an array for different units
Step2 : Create a similar function like you've seen in the sample project before
Step3 : Call the function when you're changing the text values, setting the function parameter as the number you want to be "numberized". Note that this function should return text, therefore you can only use this to display your values.
The formulas I used to extract number of 0's and first X digits are inspired from this topic:
https://www.construct.net/en/forum/construct-2/how-do-i-18/work-big-numbers-100053
I hope this helps!