Once an app is converted to apk, if the app contains sensitive data like passwords on the code, could an experienced person somehow see that password inside the apk, or converting it into an aia, or in any way?
Yes. You might use the obfuscated text
Produces text, like a text block. The difference is that th etext is not easily discoverable by examining the app’s contents. Use this when creating apps to distribute that include confidential information, for example, API keys.
Warning: This provides only very low security against expert adversaries.
So it's not 100% secure?
of course not. Anything is hackable. Perhaps encrypt?
and how do I encrypt it?
Kk! Tysm for helping
If you are that concerned, you should not store any sensitive information in your app at the start.
Create a secure and verifiable signup/signin system (so only people allowed access to the app can have it) and then this can provide access to any sensitive data to be brought in from an online resource (which could also be encrypted, and require additional effort by the user for decryption)