I'm CERTAIN that Riley Hollingsworth could answer this question best, so I can only speak from my experience of holding both Amateur Licences (all of them) and Commercial Licences (all of them):
Here's where the confusion comes in (I think.) For example, If you have a Ham Ticket, and can show that you had it before a certain date, again just for example: Extra Class that required 20 wpm code, they will use that code "element" towards your commercial telegraphy license, and you won't have to take that "element" for a second time, because after all, you already completed it.
So, all of the licenses consist of "elements." And some of those "elements" can be used to complete both Commercial and Amateur licenses.
However, AMATEUR RADIO is regulated under Title 47 CFR Part 97. But COMMERCIAL Radio is regulated under Title 47 CFR Part 90. So that's the difference and where you find the requirements for the respective "elements" required to complete a licence.
So, Setting aside some "elements" in common, the twain shall never actually meet.
With all of THAT being said: All of the question pools are published, and they "feel" pretty similar in nature. If you can pass the Amateur "Extra", then with a little study, you shouldn't have much of a problem with the "Commercial" GROL with the additional Ship Radar Endorsement, the GMDSS Operator/Maintainer, and finally the Commercial Telegraph.
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