How to copy and paste in built-in dictionary
There doesn’t seem to be a way to copy from the built in dictionary, but there is a workaround.
For the English dictionaries and thesauri, you can almost always find the same content freely online at Lexico, as the content for both comes from Oxford. (The same content even usually comes up when you search Google.)
I do this a lot actually and the content rarely ever differs between Lexico and the built in dictionary, even between the American built in dictionary and the British section on the website (both dialects are available on both the built in dictionary and the website). The website tends to have more example sentences though.
For example, gimme: