A little summary about SSL key pairs

I'm a little lazy, my understanding is just one sentence, take openssl as an example, first use the openssl tool to generate a private key, and then a series of algorithms inside it generate a public key, give the public key to others, leave the private key for yourself, and send it to others The message is encrypted with the public key, and after I receive the message, it is decrypted with the private key. I want to ask why the public key is used to encrypt, but the private key can be used to decrypt? This problem is obvious. The public key is generated by the private key. Even if it is not saved, can it be generated again with the same algorithm?