You can explore the layouts using the blueprints. Here's a website with images of her interiors and such. Here's a map giving a general idea what the geography of the classes were. I apologize, but I couldn't find a breakdown by deck. It should be easy to spot what classes go where and what is for crew.

Dining rooms: First, Second, Third

Smoking Rooms: First, Second, Third (which was actually the general room complete with a piano)

Staircases: First, Second, Third

Representative staterooms (no specials): First, Second, Third

First class passengers could theoretically go to second class, but not reverse. The third class would be absolutely separated due to fears of disease, so there was no mixing there even if a first class passenger wanted to. First class passengers also were not able to access crew-only areas.

The second class was the smallest of all groups on the ship as the middle class wasn't that prominent back then. Their rooms also ranged from being really nice to being about third class quality. There was still a divide between them and first class. For example, the A La Carte restaurant and Cafe Parisien were added onto Titanic (thus removing a bit of the Second Class open promenade deckspace) because the First Class passengers in the A La Carte restaurant didn't like the second class passengers peeking in on Olympic.

First Class passengers were the only ones who had private washrooms (as you can see on the blueprints). You can also see that not all of them had their own private washrooms. That's because there is a massive range in the cost of first class tickets. If you want "entry" first class (because you're only there for 6 days and don't see the point when you aren't going to be in your room a lot anyways), you can pay 26 pounds 6 shillings (about $3500 today) and that will get you E-25 (as in E-Deck) with a bunkmate. If your money causes your leprosy to worsen and you must get rid of as much as possible immediately, you can spring for the 32 x 50 foot ensuite for over $100,000. To use for 6 days. Per person.

Whatever class you were a part of, you got access to all the amenities of that class. There wasn't much room on board to state "if you purchase Diamond Tier tickets, you get X!" You'll notice by looking through the blueprints Titanic is not a spacious ship. You pay a lot less to get a lot more today. Try telling someone they should pay $100,000 for that ensuite today.

They did have to pay to use certain facilities (A La Carte restaurant or Cafe Parisien in particular). Food and drinks are limited and setting aside materials for special orders is no small task when you could have to nourish 3,500 people for a week with no chance of restocking.