How do you make warp pipes work so they can warp you to different parts of the stage or to a new environment completely?
The ability to warp to subworld with pipes is unlocked pretty late. Once unlocked, drag Mario to any of the pipes and place the pipe anywhere in the subworld/overworld. There is a limit of ten warp pipe connections per level. Warp pipes are best used to transition between overworld and subworld, but they can be also used if you don't want to use doors.
I've also played all of your levels available, and here's my thought on each level.
The first level is of course meant to be a tutorial level, but this one definitely have some improvements to do even if you don't have all of the necessary tools right away. For a tutorial level, the clear rate is ironically pretty low, sitting at about 25% when I last checked. Why that low? There's a few level design issues that you might haven't seen yet came into the play.
First up, there is zero powerup in this level, therefore players have to clear the level without being hit even once or else they get sent at the very beginning of the level every time they die. This is especially true when your levels are long and Super Mario Maker lacks checkpoint, as the further the player dies, the more aggravated they will become. Since a lot of people don't have much patience, they would easily quit the level. It would be nice to add at least a Super Mushroom near to the starting point and also after a tricky segment of a level to encourage them to go on further, because it grants Mario the ability to take one more hit before shrinking back to its weakest form. And that definitely reassures players seeking for more room for errors.
Second, some parts of the level asks you to perform certain moves, but the level design doesn't prevent players from not heeding your advice and let them proceed in ways you wouldn't expect. For example, the running portion can be beaten by walking instead of running. You could at least leave a small hole for every 2 squares of ground (without them being bottomless) and you'd already have a level design that encourages players to run that section, although a super-fast conveyor belt with super-low ceiling would do a much better job on teaching that particular move's usefulness.
It's also worth noting that the run and jump segment can be beaten by just walking and jumping, because walking at full speed still gives the same jump momentum as running in full speed. The difference between walking jump and running jump is that the latter lets you traverse a significantly longer gap than the former.
Third, Spin Jump isn't what I would call a basic move. In fact, it's an advanced technique available on both Super Mario World and New Super Mario Bros. U style that not a lot of people actually knew its existence, save for a few later levels. The move is tricky in its nature and the games that featured this move didn't emphasized it often enough for it to be second nature for most players. It can take a longer while for them to master the intricacies of spin jumping. Therefore, I suspect a lot of players died between the tight Spin Jump section by accidentally hitting a Goomba afterwards and the stair of Piranha Plants near the Goal Pole. Speaking of the staircase, there isn't enough leeway to avoid inadvertently hitting one of them and requires the player to perform very precise spin jumps. That is already a pretty hard section, let alone without a powerup for an otherwise easy level. And that is coming from someone who is very familiar with spin jump.
The second level isn't really memorable for me. Just like your first level, it has the same problem of players not heeding the directives you want them to follow, since players could skip the entire mini-game with the Cheep Cheeps giving them a wide opening, thereby ruining the whole point of the stage. Thanks for letting me know that bowling is all about hitting Cheep Cheeps one-by-one with fireballs and guarantees the players a full strike every single time! Also, the stage suffers from some repetitiveness as the same exact bowling segment occurs four more times. The only good thing about this stage is that it isn't even challenging at all, which should be a fine stage for those playing 100 Mario Challenge on Easy.
So yeah, it's nice to be creative and all, but you'd have to make sure that your ideas will stick pretty well whenever you want to. And most importantly, make sure it's enjoyable for many players who would stumble in your levels. Since you were hankering for more course elements, have you tried spamming tonnes of blocks every time the game says more of them would arrive at certain date? It'll make them happen like right now.
