The professional way would be for them to work on a copy of your website set up in their own servers.
This means that you should give them all the ftp files + the related DB. Then they work on their website and when the work is done you can update your website.
If you want professionals working on your website of course you should be able to trust them. If you don’t trust them move on to a more reliable provider.
Another way would be to give them access to your admin and in case something bad happens you can just contact your hosting and ask to revert to a backup.
Before starting of course you should do that backup.
But in my opinion this is not professional at all and should be avoided.
I would also avoid to hire them just to use Elementor. It may be a waste of money. If they customize your website with elementor + css and maybe some JS to actually improve it and make it unique that’s okay, but if coding is not involved hiring an agency is a waste of money. You could just “hire” a virtual assistant able to use Elementor.
But again, I would still avoid it, because to properly improve a website you will need a professional to keep the performance on check.