the data you need to put in your file is provided by the ad service.
so you will find it somewhere in the admob/tappx/whatever console.
put them all in 1 file and upload it to the root of your site.
admob will use your developer site setting in google dev / apple connect to check if the file exists.
I had a link to a facebook app page in my apps profile for support reasons now the problem is that I can't upload that file there in the root so I have to change things.
for those who don't have a website/domain yet this means extra costs unfortunately. (domain name & hosting, unless you use some free service)
there are some free services like tappx has where you can put your file but I thought I read somewhere that these will be or are being ignored by google.
(maybe this was for the privacy policies, I can't remember exactly)
so I guess you better spent €40/year to have you own domain name & minimal hosting package as it looks more professional than something like myname.00hosting.com