If I understand your question correctly, you're looking at two potential key misuse points here:
Developers can accidentally use production keys while developing your app - this is easy enough to solve by storing your keys in your CI pipeline (assuming you've got one) and having it inject the correct secret into correct environment configuration. Some tools that can be of interest: Octopus, Hashicorp Vault. Developers then will only have development keys in their code base. Bear in mind - if you are using version control systems - it's not going to be enough to just delete your production codes and add a new commit - there are tools that let you search your commit history for accidentally exposed secrets, so you'll have to change your keys
Users can reverse-engineer your app and extract keys from the code. This one is harder to tackle as it hugely depends on the OS, version and how you work with secrets. Generally you would like to avoid storing secrets in your app at all and rather obtain them when authenticating users. After that - you'd leverage your target OS secure local store features (keeping in mind that even that does not guarantee 100% protection). For 3rd party access, consider proxying the requests through your server to conceal the keys Some more inspiration can be found here
UPD to clarify your concerns re user interaction, consider this simplified workflow:
1) User issues an unauthenticated request to your backend /authorise
endpoint that will check username, password and return a token1 (preferably JWT)
2) your app stores this token1 in local store on the device - this is the only secret that user will ever have access to and it's specific to that user
3) Your user makes an authenticated request using token1 to your /3rd-party-api-proxy
4) your server will validate the token1 from step3 and make the actual request to 3rd party using token2 that you never exposed.
5) your 3rd party request succeeds and you return data to the user.
with this flow your token2 never gets exposed and you always know which user requested access to 3rd party API (you can add logging, audit, and what not). There;s plenty of articles in the Internet on how to architect this thing, I just outlined very basic concepts here, hoping this gives you some food for thought.