I'm new to Adobe and wondering when I see a page like the screenshot I provided, is it fully free? Any hidden fees?
When you search for Assets in the Buyer portal there is a drop down selection in the search bar. If you choose "free", the selections are filtered to show you only Free images. There are no strings attached and no fees when you download such images:
Additionally, if you wish, Adobe also offers a Stock 30-day trial subscription that allows you to download up to 10 standard assets for free during that period. If you don't cancel that trial within the first 30 days, you are on the hook for the subscription cost, so read the T&C's carefully and make sure you know what you're signing up for:
Free assets are part of a collection that Adobe makes available for free. This does not mean, however, that they are in the public domain. You licence the assets and are bound to the terms of that licence. However, free assets may be used in commercial projects the same way as paid assets with the same licence may be used.
But yes, free means you do not have to pay for the licence to use the assets. And the licence has a lifetime validity. Even when the assets return into the paid pool, you may still continue to use the asset licenced for free (in line with the licensing terms).