What certificates are used to encrypt data when sending and receiving information on openvpn?

Question

openvpn

Hello and in advance I’ll thank everyone who will help me, I understand how asymmetric encryption works open private key all things, I’m interested in open vpn there are two certificates, a client certificate and a northern certificate, and private keys to these certificates, The question is what for when the client sends data to the server with which certificate the data is encrypted, and when it receives data from the openvpn server, which certificate encrypts the data, then where is the private key of the server and client certificate stored? Thanks