What is the difference between systemctl start [service] and systemctl start [service].service?

No difference between them.

.service is the unit file containing all information necessary to control the service (start, restart, stop, logging etc.)

Yes, apache2 is the shorter version of apache2.service for the question's context.

First link with Google systemctl start why .service has the answer. Here the copypasta :)

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In systemd, the target of most actions are “units”, which are resources that systemd knows how to manage. Units are categorized by the type of resource they represent and they are defined with files known as unit files. The type of each unit can be inferred from the suffix on the end of the file.

For service management tasks, the target unit will be service units, which have unit files with a suffix of .service. However, for most service management commands, you can actually leave off the .service suffix, as systemd is smart enough to know that you probably want to operate on a service when using service management commands.