Let Tomcat is download and installed under /opt/tomcat.
Also, let tomcat be a non-provileged user under which the server will be running.
We assume that we keep server's binaries under /opt/tomcat and we will create a server instance named foo under /var/tomcat/ (carrying its own conf, logs, webapps, work, lib directories).
See also https://dzone.com/articles/running-multiple-tomcat.
Create a template service unit file at /etc/systemd/system/tomcat@.service: