How do you get a block from a block in creative?

They use an action described in the game controls as 'pick block'. By default it's the middle mouse button (“mouse 3”).

They use 'pick block' which is middle clicking on a block.

you need to simply click on the block and move it. I hope this helped you!

When you use a mouse, press down the scroll wheel with an open hand, whilst having your mouse on the block. It will give you the block.
But when you do it in the inventory, it gives you a stack.
But you have to be in creative.

To pick up the block you are looking at on PC/Mac, press down the scroll button. There are no current ways to collect the block you are looking at on the Console editions, or Pocket Edition.

You can change your key binding for pick block in the settings. They added pick block to console versions as well now. If you do it in creative you get the block but in survival, it takes it from your inventory

It is called Pick Block, and by default it is set to MMB (Middle-click).

In creative, it adds the block to your inventory.

Many people do not know, though, that if you're in survival, you can also use it. When used in survival it puts the block in your hand if you have it in your inventory. For example, if I look at andesite and have andesite in my inventory, I can pick block the andesite and it will move straight to my hand. If you look at a block that is not in your inventory and try to pick block it, nothing happens.

You can also copy the block along with the data. Pressing CTRL + MMB (change MMB with whatever you bind it to, but CTRL cannot be changed) will copy the block along with its data. For example, if there is a command block with a command in it just pressing MMB on it will copy an empty command block. Pressing CTRL + MMB will copy the block along with the command inside of it.