Thanks for reaching here! We recommend you to starts with Forms Recognizer product: https://learn.microsoft.com/en-us/azure/applied-ai-services/form-recognizer/overview?view=form-recog-3.0.0.
You can use the recognition models that exist already or use a custom model and train it. With Forms Recognizer Studio you can see how each model processes data and how it maps key pairs.
Further to integrate the fields mapped to Cognitive Search, you can refer this document link for custom skill: https://learn.microsoft.com/en-us/azure/search/cognitive-search-custom-skill-form
Please let us know if further query or issue remains.