All Recommendation related APIs can be found in the documentation here <-- check it out!
And as always, please let us know your feedback/ questions so we can ensure that all of your queries are addressed adequately!
could you please tell me which API to use in order to fetch recommended products that match to a given product item?
So for example, I assume providing product 123 will need to fetch associated activity / criteria first to then be able to fetch the recommended products by this product 123 / activity 234 combination.
But please give me hints which route to follow.
That would be in the Delivery side of the API. After having set up a Recommendations type activity in Target, you can then qualify for the activity by looking at this:
Target API v1.0
I would categorize the above (Adobe Target Recommendations API corrected) link to be the admin side of dealing with Recommendations. Most of those endpoints mimc UI functionality within the Recs portal.