If they only make a donation without availing a ticket, they won't be able to enter the event as a donation will not issue them a ticket. They will still need to get a ticket to enter your event.
When setting up your ticket, you can either have a paid ticket type or a free ticket type, and there is also an option to add a donation option while buying a ticket.
Go to Setup Tickets
Click Create a Ticket
When you add the donation option, it will appear at the bottom of the ticket selection when they register for your event.
If an attendee avails of a paid or free ticket and donates, they will receive a confirmation letter that indicates the number of tickets that they have bought and that they have also made a donation.
If an attendee just donates and does not avail any ticket, they will receive a confirmation letter that says 0 x Ticket. They won't be able to enter your event because they don't have a ticket.
To make sure that they can enter the event, you can issue them a free ticket by registering them through your Add Attendee tab.