It’s a lot to ask us to watch a 15 minute video and try to figure out which part you’re referring to.
I would suggest using Firebase for this, if you can. Firebase is intentionally designed to allow users access (via rules) to only their own data.
If you can’t or prefer not to use Firebase, then you would store the user’s ID in a column and each time you want to access only their columns, loop through the ID column and save the rows that match the user’s ID to a variable. Then use that variable (list of rows) throughout the rest of your code.