If I buy X of an ETF, do I own X of underlying stock?

Let's say that I buy $100 of an ETF, like VOO or SPY.

Do I own $100 of underlying stock?

Or it is possible that the ETF has a value that is much higher than the actual stock?

For example, I buy for $100 of an ETF, but that corresponds only to $80 of actual stock.

Is there any way o ensure that there isn't too much difference between the direct ownership of the stocks and being part of an ETF?