I would like to know what would be best, use a spreadsheet to hold all the data, or separate them into multiple worksheets.
That will really depend on the scope / complexity / overall structure of your intended applications.
For example, if you envision a suite of applications with differing / distinct purpose, it may be logical to have a separate workbook for shared tables such as Staff, Projects, etc. Conversely, if your application is simple in nature and stand-alone, it may be logical to keep it all in one space.
If you are planning on using Google Sheets specifically, it is also good to be aware there is a maximum cell limit (2 million). You can sort of skirt this by breaking your data up into multiple files - if you think you would ever get close to it.
Adding on to @Jonathon, there are a few reasons why you might split your data into multiple worksheets.
It could be a way to improve the scalability of your app. See this article for details: https://help.appsheet.com/performance-scale-reliability/scaling-to-large-data-sets/scaling-using-data-partitions
Another reason is because it logically makes sense to have separate tables. For example, take a look at this “Item-Detail” pattern that comes up regularly. https://help.appsheet.com/app-design/modeling-tables-and-columns/the-item-detail-data-pattern
The Order Capture app is a good example of the item-detail pattern: