I'm returning to this thread, having played Super Mario Galaxy fairly extensively now. As a first-time player, I'm absolutely astounded by the gameplay. Some people may remember that "Tiny Planet" was a set of videos that went viral on YouTube about 4 or 5 years ago.
E.g. this. This is probably the best way to describe levels in Super Mario Galaxy. Every level in the game is a series of small planets that you have to negotiate. Each of these planets are 3D wrap-around.
Here's one of the smaller planets zoomed out, early on in the game:
If you go halfway around the planet, you end up "underneath" it but you don't fall of it because of the mavity. It takes some getting used to! Note that some areas that aren't planets are flat and it is possible to fall off it and lose a life.
Some planets are multi-tiered, so you start off on its surface, and once you've cleared that, you descend beneath the service and start clearing that:
Some locations are inverted, so that the 3D environment is an enclosure but still wrap-around. These are mostly seen as underworlds via the green pipes which is common in most Mario games:
Some planet shapes are downright weird e.g. this one is in the shape of a question mark (?). The mavity still plays its role anywhere along the planet's body:
What I've written here might be irrelevant to most Nintendo players, so it's really for those who never got the chance to play it on the Wii. It's easily my favourite game of 2020. There is also a character that is new to the Mario games called Rosalina and she is kinda like your mentor. She advises on how the planetary systems work and how to keep building on the space station where the base game takes place on. In between levels, she also reads stories out to the player on her upbringing and how she became to be. I actually first discovered her in a separate game called Super Mario Party (as a playable character or as an NPC), so I dug up Super Mario Galaxy (from the 3D All-Stars 3-game pack for £40) and that's where I am now. All screenshots are from my Swtich. The Tiny Planet pic at the top was taken from Wikipedia.
My current progress is that I've just beaten Bowser Jnr for the 2nd time where he fires cannons from a ship and you have to fire green Koopas at his ship. 3 or 4 times is enough to sink his ship. I've no idea how far I am in the game?! I have 38 yellow stars and 1 green star btw.