Bank assessment, question about stress testing

Quick question about Stress testing.

I understand that when applying for a loan, banks would use a rate that is higher than the actual interest rate to assess your serviceability, such as if the actual lending interest is 4%, banks might internally use 6-7% when analyzing your application, as a way to ensure if the interest rate hikes, you will still be in a position to be able to make repayments.

However, my question is… will they stress test only the loan you are applying for? Or will they stress test all of your properties in your portfolio which has a loan?

Say you already have a portfolio of 5 properties and you are buying the 6th, and interest rate is 4%. Will the bank apply 7% testing against your 6th property only? Or will they applying 4% against the other 5 in addition to that 6th property you are buying?

I am just trying to think for people who have 20 or 30 or more properties, if the bank applies stress testing against all those 20-30 properties in their portfolio, then the amount of “compensation” required to satisfy such stress testing becomes exponential, and unless their income is also increasing exponentially (which won’t happen unless you are talking about business income rather than personal income), then how are those people ever making it past bank assessment?