Yes, all entries are stored in the database. You can find the database schema in the Formidable Knowledgebase. Are the entries encrypted? No. There isn't any form builder that encrypts out of the box. You need an add-on. https://formidable-masterminds.com/downloads/ff-encryption/. In general, it's a bad idea to store tax ids in any system without much deeper security than WordPress/Formidable can provide. You need to make sure your server's security is bullet proof and you're not violating any data privacy or other regulatory compliance obligation by storing the data in your database.