A Linux system is build around many tools. Systemd replaced the init + gradually many othe parts, it can now handle logs, schedule jobs, manage containers, be the local dns resolver, etc. All the bricks that makes a linux system without systemd still exist. All those additional functions of systemd are optionnal but afaik they depend on systemd to handle the init to work. This is the reason openrc is often seen as a replacement as you first need to change the init to replace other functionnalities anyway.