How can I get a Google Ads API Developer Token
By current Google documentation
it's in the "TOOLS & SETTINGS > SETUP > API Center" but now "SETUP > API Center" is not there.
What is the current way to get a token or what page is the "API Center" currently on?
you will only see the API Center if the account is a Manager account. You probably are trying to see it from a Client account. Go here to create a Manager account.
If you want a demo app that is integrated with Google Ads, check out this video tutorial and check the description section for more resources such as frontend code repo, backend code repo, technical guide explaining how to build every feature. Video tutorial --> https://youtu.be/WvP3vtc_o7Q