Do the developers see that Im using the same device to login from different accounts?

Apps can have enough saved state to be able to identify specific devices, yes.

As for what else an app developer can determine for a particular app, check with the app developer.

Blockchains are inherently transparent, and blockchains can be reviewed anew whenever additional details, identities, server logs, subpoenaed business or tax records, and related context become available. These re-scans of the blockchain might occur months or years later.

Frauds, scams, embezzlement, Ponzi schemes, “rug-pulls”, and related are endemic in cryptocurrency, of course.

AntoniCiorici wrote:

there is a lot of scam if u rush and don’t do a proper research. That’s for sure.

Exactly. Your concerns are valid. That's why Apple doesn't allow developers to identify individual devices. That's the rationale for most of Apple's App Store rules for developers - to protect Apple's users and their private information. But crypto is well-known for attempting to bypass those kinds of privacy restrictions. Crypto users are particularly targeted for malware attacks due to the potential for direct theft of cash.

But because crypto is such a popular and lucrative domain for scams and malware, there's no way that Apple can stop them all. Plus, various government are now trying to directly prevent Apple from stopping and restricting these kinds of scams.

This means that there is non way to tell what is or is not possible inside an app, especially when apps are going to be coming from "alternative" marketplaces or even side-loaded. You have to assume in the future that all those restrictions are gone. Any research that you may have done before will no longer be valid.

Anything involving "crypto" is a scam. You should assume that any apps are using any and all kinds of nefarious methods to steal money.