I thought most "garbage patches" in the ocean were actually made up mostly of microplastics.

I can't find a great source for this now, but a quick Google gave https://education.nationalgeographic.org/resource/great-paci... which says:

> Even satellite imagery doesn’t show a giant patch of garbage. The microplastics of the Great Pacific Garbage Patch can simply make the water look like a cloudy soup. This soup is intermixed with larger items, such as fishing gear and shoes.

Probably slowly breaking down into microplastics though

"For many people, the idea of a 'garbage patch' conjures up images of an island of trash floating on the ocean. In reality, these patches are almost entirely made up of tiny bits of plastic, called microplastics. Microplastics can’t always be seen by the naked eye. Even satellite imagery doesn’t show a giant patch of garbage. The microplastics of the Great Pacific Garbage Patch can simply make the water look like a cloudy soup. This soup is intermixed with larger items, such as fishing gear and shoes.

"The seafloor beneath the Great Pacific Garbage Patch may also be an underwater trash heap. Oceanographers and ecologists recently discovered that about 70 percent of marine debris actually sinks to the bottom of the ocean" (https://education.nationalgeographic.org/resource/great-paci...)