You apply for transitional or preliminary medicine interships through ERAS like everyone else does. As for which is better I think doing a prelim medicine year will be more valuable to your ophthalmology training. A good fundamental background in medicine will help you immensely in Ophtho. Depending on the program you apply to the number of electives will vary. I would recommend a prelim medicine program with electives in neurology, radiology, dermatology. A medicine year will also help you on the okaps on the medicine section. Try to chose a prelim year where you have a lot of input into patient management this is the only way you will remember your medicine come your ophtho residency. Most optho programs have inpatient services, and it sure is nice to remember how to acutely manage severe hypertension and diabetes while the patient is on your service inhouse.
