“Where does it say a democrat primary only?”
This might help.
https://www.michigan.gov/documents/sos/Voters_QA_MIPresPrim_516112_7.pdf
It is neither a GOP or DNC mixed party competition. As it says in the site, before you enter the booth, you select a party and vote off their candidates, only in the party’s competition. Of course, unless you want to write in a GOP candidate, if you select the GOP ballot by party, Trump will be the only one on the ballot as no one else has filed to my knowledge.
rwood