Giving secure access to a 3rd party

Apologies for what is a very basic query but I’m a complete novice. I have a 3rd party doing some template revisions for me. What is my most secure way of giving them full access to carry out the work on a temp basis? Also if they have full access, do they have sight and access to my payment details on my account. Appreciate some guidance.

Thanks

The third party honestly should be doing the theme build in their own lab. And then when everything is over, help you migrate or give you documentation on how to build the pages etc.
They shouldn’t need access to your site.
If it’s Spiritix, Bright Themes, or Cathy; these are exceptions to my statement. These are reputable and trustworthy individuals / organizations for Ghost and they will not hurt you.

Much appreciated, thanks for taking the time to reply.