Why can't I see all of my site/org custom fields in my sandbox?

[Danno]

I'm creating a project in the sandbox and do not want to create a bunch of new custom fields. I know of some existing fields that I would like to use but they do not show up in the sandbox project. Why?

I think some of the fields I would like to use might be associated with archived projects. Could this be the cause?

[Mikael Sandberg]

Is your project next-gen or classic? Next-gen projects cannot share custom fields, that can only be done if you create a classic project. If you have a classic project and then you first have to add them to the screen used by the project for them to show up. All your custom fields (excluding the ones created in next-gen projects) are available if you go to Settings > Issues > Custom fields.

[Danno]

@Mikael Sandbergthis is a classic project. I'm working to eliminate Next Gen from my site.

I think you are missing the point of my question. I have 9 pages of custom fields on my current site. I am unable to see any of them in the sandbox environment to add to my test project. I wanted to know why this is the case.

I am trying to not add any new custom fields. I need to thin out that list especially for a site as small as ours.

I'm beginning to think I need to export them somehow and then import them into the sandbox environment but I can not find any information that tells me this is the case.