Somebody get access to my server. Pls help me to understand how it happened

Hello. DigitalOcean support banned my account because my server was trying to get acces to another one. Here is log from target server:

Oct 8 17:09:51 c-3po sshd[31374]: Did not receive identification string from 82.196.2.248 Oct 8 18:49:19 c-3po sshd[7318]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=82.196.2.248 user=root Oct 8 18:49:21 c-3po sshd[7318]: Failed password for root from 82.196.2.248 port 36473 ssh2 Oct 8 18:49:22 c-3po sshd[7322]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=82.196.2.248 user=root Oct 8 18:49:24 c-3po sshd[7322]: Failed password for root from 82.196.2.248 port 36737 ssh2 Oct 8 18:49:25 c-3po sshd[7324]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=82.196.2.248 user=root Oct 8 18:49:27 c-3po sshd[7324]: Failed password for root from 82.196.2.248 port 36868 ssh2 Oct 8 18:49:28 c-3po sshd[7326]: Invalid user abcs from 82.196.2.248 Oct 8 18:49:28 c-3po sshd[7326]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=82.196.2.248 Oct 8 18:49:30 c-3po sshd[7326]: Failed password for invalid user abcs from 82.196.2.248 port 37010 ssh2 Oct 8 18:49:31 c-3po sshd[7328]: Invalid user admin from 82.196.2.248 Oct 8 18:49:31 c-3po sshd[7328]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=82.196.2.248 Oct 8 18:49:33 c-3po sshd[7328]: Failed password for invalid user admin from 82.196.2.248 port 37212 ssh2 Oct 8 20:54:32 c-3po sshd[17688]: Invalid user asd from 82.196.2.248 Oct 8 20:54:32 c-3po sshd[17688]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=82.196.2.248

As I understand someone get acess to my server. I’ve used password protected ssh key when created server. I’ve got few questions:

1. How can I detect process that sends this auth requests?
2. How can hacker was able to get access to my server? How can I figure it out?
3. Is password access to server enabled when ssh key is used?