Yosef Karo

“ Paragraph 6 — A lefty who does all his work with his left hand puts his Tefillin on his "left hand" which is the right hand of all people. And if he uses both hands, he places them on the left of all people. If he writes with his right hand and does all other work with his left hand, or if he writes with his left hand and does all other work with his right hand, there are those who say to put the Tefillin on his weaker hand because we "need the weaker hand." ”

“ It is necessary to place the tefillin at the top of the bone closer to the elbow, but not in the [upper] half of the bone close to the armpit (SM"K) . An amputee that does not have a hand, but only an arm, should put on [tefillin] without a blessing (Tosfot chapter Hakometz writes that an amputee is obligated and in the Or Zaruah it's written that he's exempt) . ”