The journal holds everything from every unit, which makes it useless without filters and extremely good with two of them.
$ journalctl -u php7.2-fpm --since '10 minutes ago'
$ journalctl -u nginx --since '2018-11-14 14:00' --until '14:30'
$ journalctl -p err -b # errors, this boot only
$ journalctl -f -u app.service # follow
$ journalctl --disk-usage--since accepts natural language, which is the part people do not expect and use constantly once they know. -p err filters by priority and cuts the volume by an order of magnitude during an incident. Worth setting SystemMaxUse in journald.conf deliberately: the default is a percentage of the disk, and on a large volume that is far more journal than anyone will ever read.