I was wonduring how can i build cool stuff like keralis does in youtube?

If you are building a city in minecraft, I would recommend structure and detail build tutorials. Also, you can just build out of the blue like Keralis, it took some practice to understand structure and detail. Examine the building he makes, and practice on making it from scratch (a little hard). Here are some tutorials that might help you out.

Try to get inspiration by looking at houses in real life or in-game. When you get your inspiration to build something, build it.

Play "The Sims". You can pirate it if you choose. And from there, you design an interior and floor plan, etc etc the works, then you use that as a "blueprint" of sorts to build the thing with, if you're building modern architecture.

You can also look at pictures, draw out the plan on MS Paint/graph paper. You should also pay attention to scale as well. If we use the player (aka Steve) as a unit, the player is 1 Steve tall, which is 2 blocks. Since Steve is generic, he is also the average. The average Human is 5 Ft 5 Inches (taken from data that I skimmed on Wikipedia, lots of people were around the high end of 5 Ft and lots were either 5 ft 5 inches or smaller this included females as well).

This makes Steve 5 feet 5 inches. If a building is 11 Ft tall, it's 2 Steves high, which is 4 blocks. The Eiffel Tower is 986 feet. This makes it 182.030769 Steves tall, but rounded up that makes 183 Steves tall, or roughly 364 blocks tall. Certain versions of Minecraft only allow up to 245 layers. If each block is one layer, then you'll need 365 layers to build the Eiffel Tower, with one layer/block of "free space".

Just like the two answers before, you just have to understand building structure. Building a city is the hardest thing i have come to try to do in minecraft. I do watch lots of tutorials and they do help alot. For example, i saw a lighthouse at a seafood restaurant when i went to a beach once that i really liked. Since i was still a minecraft noob, i watched a video on how to build a lighthouse in creative and changed the design to match the one i saw in real life. I think i did pretty well! Of course light houses arnt the hardest thing in minecraft but i hope you get what i'm trying to say. Try and get your inspiration from the world around you. Take in the details of how it looks and try to incorporate that into your buildings!