Your question is unclear as it seems like you are asking about Yosemite National Park, not Yosemite Valley. The valley is less than 10% of the park.
If you are asking about Yosemite Valley, no, there is no backcountry. If you are asking about Yosemite National Park backcountry, it gets more complicated.
"If there is an emergency then I always have Internet access." In general, no. There are a few spots near the main roads where there is cell service, but that is not backcountry. In the real backcountry, expect no service at all.
"Can you camp anywhere like you can in the backwoods?" Sort of, not exactly. You can camp in the snow in lots of areas. However, you still have to get a free wilderness permit from the rangers, and they can tell you where camping is prohibited.
"Are there other people there?" Yes, a few. As long as you are within five or six miles from a main road, you may see people or new tracks. Once you get beyond that, most of the tracks are from animals, not people.
"What are the roads like?" The Tioga Pass Road is generally closed and unplowed through winter (although it has been open unusually). That makes a good route for easy navigation for skis or snowshoes. The Glacier Point Road is generally closed and unplowed beyond Badger Pass Ski Area. That makes it a good jumping off point. Note that the road to get up to Badger Pass is often an R3 restriction for autos (chains required _even_ on a 4WD vehicle). The roads in Yosemite Valley are generally open and a bit slippery with snow.