Start with the known limitations for google sheets.
Do the math and find out how many google account spreadsheets and rows you will need to not reach max.
Create the relative amount of glide app you will need for each spreadsheet.
Create a main webiste that will divide and funnel your 100million users into the respective app according to some type of question, Location, Age, Gender etc. (Gender alone knocks you down to 50 million ( Age you can get down to another 10 million per age bracket). You may only need about 5 spreadsheets if you play your filters right.
After you have captured your data from 100 million users, Run a query with filter to combine only the relevant data you need from your 100 million user inputs
Tada there you have it. Hypothetical solution for your hypothetical app.