It depends where you are. It depends how they’re designed. And it depends how much luggage you travel with.
Some have rooms with bunks lined up next to one another.
Some have private bedrooms that lock and shared bathrooms down the hallway.
In Iceland, our hostel, Heraosskolinn, was nicer than some hotels we visited. Though we did have to use a shared bathroom. A good question to ask when booking, how many rooms/people share the bathrooms. Because the last thing you want is lining up to shower and wasting precious time you could be using to explore.
In New Zealand, we rented a room at Funky Green Voyager with two beds with a door we could lock. We shared a bathroom.
Honestly, these were both amazing experiences because we met so many incredible people from all over the globe.
If you have a suitcase and decide to stay at a hostel where you share a room with other travelers you don’t know, think about where you will keep your luggage secure while you go off exploring for the day.
If you have a rental car, you can secure your things in that. But bottom line, if you don’t travel light a shared room at a hostel is likely not your best option.