The Schiphol website probably explains this best. https://www.schiphol.nl/en/page/welcome-back-transfers/
CURRENTLY, it says this:
Can I transfer at Schiphol? Yes, you can transfer at Schiphol if you are coming from a country or area that is considered safe due to low risk of coronavirus infection. If your onward journey is within 24 hours and you do not leave the airport, you are not required to present a negative test result for your transfer. However, you do need to bring a completed health declaration with you. This applies to all travellers aged 13 and over. Please check the requirements of your final destination to ensure you comply with the entry regulations of that country or area. You may be checked for this when boarding your next flight.
Fully vaccinated You are also allowed to transfer at Schiphol if you are fully vaccinated and you are able to show valid proof of vaccination. If you stay at the airport during your transfer, you will not have to do any extra tests. However, do check what the coronavirus rules are in the country you are travelling to. If you are travelling from a very high-risk area, you may be required to show a negative test result before boarding the plane.>
Mandatory quarantine declaration The quarantine declaration is mandatory for all travellers from very high-risk areas, regardless of whether they are making a transfer or staying in the Netherlands. You can print, complete and sign the form or use the digital quarantine declaration at quarantinedeclaration.government.nl. On the form, you indicate that you are transferring at Schiphol and that you are therefore exempt from quarantine in the Netherlands. You can find a list of safe, high-risk and very high-risk areas on the Dutch government website. Check the website to find out what applies to your journey.
Do I need to take another test when transferring? No, that is not necessary. If you stay at Schiphol during your transfer, you do not need a new negative COVID-19 test result. The test result you had upon arrival at Schiphol remains valid during your transfer, including any potential delays. If you leave Schiphol during your transfer, this counts as a new journey and you are required to present a new negative test result. You can find out more on the Dutch government website.
As of today, as you meet the fully vaccinated criteria, it appears you do not need a test. This could change and is also dependent on what the entry requirements are for Germany. As you will have to go through passport control, odds are you will be asked for proof of vaccination status, at a minimum. And yes, the NL websites can often be confusing and conflicting...