Why header and payload in the JWT token always starts with eyJ

JWTs consist of base64url encoded JSON, and a JSON structure just starts with {"..., which becomes ey...when encoded with a base64 encoder.
The JWT header starts with {"alg":..., which then becomes eyJ...

You can try on this online encoder and enter {"alg" and click on encode. The result will be eyJhbGciPSA=