Hi @Miguel , (Note: I'm not sure why Q&A is tagging your name as Castansa when your profile shows Miguel)
There isn't an Azure certification that is specifically focused on B2C or custom policies, but the Microsoft Azure Administrator (AZ-104) and Microsoft Azure Architect Design (AZ-304) certifications cover some level of B2C information.
The best way to learn about custom policies though is to go through the custom policy starter packs and experiment with them. We also have the B2C custom policy overview guide and the custom policy series.
There are some really good walk-throughs on Youtube as well, such as this tutorial that goes through the starter packs and a hands-on lab. I'm also tagging @Jas Suri in this thread as he is a subject matter expert on B2C custom policies and may have additional resources to share.
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