@JorgeV ... Welcome to Webmasterworld!

Sadly I don't have any answers as I have nothing on my sites that intice the mediapartners-g to visit. Perhaps others will have more information to share.

If you have created and uploaded an "ads.txt" file, the mediapartners-google bot should be able to sort your account. If you have not visited your AdSense account for awhile you can see right away in your account interface whether your sites are ready.

Hello-

Thank you for your comments. It's not a problem of access, or allowance. I do have the ads.txt file, and all is fine. It's just that, I have a very old site, running since 1993, and over the time, it reached millions of pages, and on pages which are not frequently visited, the first visit , since a while, is not showing ads, then some minutes later, ads are available, after the adsense bot's hit.

Here are some answers to your questions, based on the information I had when dealing with this issue in early 2018:

- does it mean that, until a page is fetched by the mediapartners-google bot, no adsense ads will be served?

Basically yes. If mediapartners-bot has not indexed the page no ad will be displayed. It is important to know that the Adsense index has a TTL(Time-To-Live) of about two weeks, so if a page is crawled and then not revisited within two weeks the ads may not show again.

- is it the first visit of the page, which triggers a visit from this bot? Or does the bot browse the site on its own like Googlebot too?

Each time a an ad-request is made from a page AdSense checks it's index for the page, if the page is not found then mediapartners-bot is sent to retrieve the page, the bot may also retrieve the page at other times, but this is the main mechanism.

- after a while, doesn't Adsense deliver a "white card" to a site? Like for example, after visiting xxxx pages, without encountering "bad" content, it whitelists the whole site?

There does seem to be some sort of a white-list but I don't think it is time dependent and I'm not sure how to get on it.

Now the bad news... I doubt that any of this is relevant anymore. Adsense has announced some significant changes in the last few weeks. Specifically on how they handle ad-serving and policy violations. Basically they will limit serving ads on pages that do not meet their requirements, while allowing some advertisers to opt in to showing ads on these questionable pages. This new policy completely changes how "Brand Safety" is handled. This change has gotten little traction here at WW (likely due to the labor-day long-weekend), here is the link to a thread discussing it and providing more details:
[webmasterworld.com...]

Basically, this may help you. If your site was "black-listed" on the domain level, then this change should allow ads to be served on the "ok" pages. But as with all changes only time will tell.