What these algorithms are for
SHA-256 is the workhorse of modern software: file integrity checks, API request signing, blockchain, and password hashing pipelines (as a building block inside algorithms like PBKDF2). SHA-512 offers a longer digest and is faster on 64-bit CPUs. SHA-1 is included for compatibility with older systems — it's considered broken for security purposes and shouldn't be used for anything new.
Hashing is one-way
A hash cannot be "decrypted". The same input always produces the same digest, but there is no practical way to reverse a digest back into the input. That's the property that makes hashes useful for verifying integrity without revealing content.
Note on MD5
Browsers don't ship MD5 in their crypto APIs because it's cryptographically broken. If you need an MD5 checksum for a legacy system, compute it locally with md5sum on Linux or CertUtil -hashfile file MD5 on Windows.