If you have the correct permission, Custom Fields are in the "Administration Screen (cogwheel gear icon) > Issues > Custom Fields" on the left side of the screen. Some of the System Fields are on these screen to (take a look at the tabs on the left side again).
Maybe this will help:
scroll down to :
In the table, you'll see
>Show list of fields
Open it and there are the standard fields useable by JQL.
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