JWE nimbus-JOSE authenticity and JavaScript counterpart

Question

I have chanced upon this wonderful library for java nimbus-JOSE for the use of JSON Web Encryption. This library is simple and requires few lines for JWE encryption and decryption like below shows JWE Encryption.

EncryptedJWT jwt = new EncryptedJWT(header, builder.build());
  RSAEncrypter encrypter = new RSAEncrypter((RSAPublicKey) keyProvider.getDefaultPublicKey());
  try {
    jwt.encrypt(encrypter);
  } catch (JOSEException e) {
    throw new RuntimeException("Failed to generate encrypted token", e);
  return jwt.serialize();

However, I had some questions regarding this library even after reading its javadocs and documentation. With respect to JWE and this library specifically,

1. After decrypting the JWE, how does one validate the authenticity of the JWE? There are no "signatures" in a JWE, so how does the 5 parts of the JWE help to check for validity? Is there any support for this in the library?
2. Other than the header and the claims, are the other fields in the JWT randomly generated by the library? (I was not able to find clear evidence of this.)
3. If I use alg: "RSA-OAEP", enc: "A128GCM", I should be getting a different JWE everytime i encrypt, however, I have been getting the same JWE. Shouldn't RSA-OEAP-256 prevent this from happening? What is the detailed sequence of encryption and decryption flow?
4. I have been trying to find a similar library in Javascript but was unable to find one like nimbus-jose which was for java. If anyone has a recommendation please do help below! I would like to thank anyone who can help with my queries. All inputs are welcomed. :)