Learning about Mobile App Security
For mobile security, there are a few companies offering courses/certification but they are fairly expensive. Security Compass, and Secure Code Warrior, would be my recommendations if you want to go that way. The community is split on whether that experience was positive for them.
If you want to study on your own, make sure you understand Symmetric and Asymmetric cryptography, how to use CryptoKit, how to encrypt/decrypt files using AES GCM, how to sign and verify signature of data using P256, how to generate a symmetric key using asymmetric keys using P256 ECDH key derivation.
You will also want to understand how to use Apple's TouchId and FaceID, how to use the SecureEnclave, the Keychain.
Make sure you know how to enable iOS file protection when writing files to disk (NSFileProtectionComplete) and how you can also set this by default using App Entitlements.
There's also Runtime Application Self-Protection (RASP), TLS Pinning, making sure you account for TLS bypass attacks.
Understanding mobile security also means knowing which tools the hackers use, so get familiar with MobSF, Frida, Ghidra, and Radare2. Be aware that like most topics, this is a rabbit hole from which you may never reach the end.
Other stuff you may want to brush up on:
User input sanitation
Data categorization (PII, public, private, classified, export controlled, etc)
Privacy cover screen
Secure text fields
API documentation (defining maximum lengths, allowed characters, date formats, integer ranges, nullable fields, etc)
Objective-C method swizzling
Red Team / Blue Team
Make sure to also watch security forums, read up on all the latest zero-days. Hackers keep innovating and discovering new ways to compromise our systems so we have to find new ways to defend our apps!
Go blue team!