documents needed to enter schengen countires

You are asking two different questions - one about entering the Schengen zone, and another about Covid requirements for different countries. The Schengen countries have common requirements for entry of visitors (i.e. Immigration). You need your national passport and you need evidence (if asked) that you can support yourself during your planned stay and you can pay for your ticket back home. You may then travel throughout all Schengen countries as long as you don't stay longer than 90 days during any 180 day period.

Your other question is about Covid documentation. This is separate from the "immigration" question. Each country has its own Covid requirements. Spain requires that you register on their Spanish health site (SpTH, with QR code, etc.) and provide vaccination information and declaration on that site. I am not familiar with the requirements for the Netherlands, but I would be surprised if they require anything more that a vaccination certificate. (That is why they don't say anything about a QR code.)

Then, when you are boarding a flight for Spain, you will need to show the QR code that proves you have entered your vaccination information into the SpTH app. I think this can be done only during the 72 hours before your arrival in Spain.

I am curious why you are not booking your flight to Spain until AFTER you land. That might make life and your ticketing more complicated.