systemd timer units are a type of systemd unit files that define when and how to activate another systemd unit, usually a service unit. Each timer unit has a corresponding service unit that it controls, and by default, the name of the timer unit matches the name of the service unit, except for the suffix. For example, a timer unit named foo.timer activates and manages a service unit named foo.service. The service unit defines what to run when the timer elapses, and the timer unit defines when and how to run the service unit. The timer unit can specify different types of timers, such as calendar-based or monotonic timers, and various options to control the frequency, accuracy, and persistence of the timer. The timer unit can also override the service unit to activate by using the Unit= option in the [Timer] section of the timer unit file.
The other statements listed are false regarding systemd timer units. Timer units are not defined within a service unit's file, but in a separate file with the .timer extension. The command executed by the timer is not specified in the timer unit's file, but in the service unit's file that the timer controls. There is no dedicated system service named systemd-cron that handles the execution of timer units, as timer units are managed by systemd itself. Timer units can exist in both the system scope and the user scope, and users can create and manage their own timer units in their home directories. Reference:
systemd/Timers - ArchWiki
systemd.timer - freedesktop.org
Working with systemd Timers - SUSE Documentation