The two main methods of placing multiple blocks at the same time in vanilla minecraft are either:
Fill Command, type "/fill x y z x2 y2 z2 block" replacing x,y,z with the start locations and x2,y2,z2 with the end location (making a rectangle area). The block should be replaced with any of the Minecraft blocks for example 'stone'.
This requires command blocks unless you want to type the command each time and relative coordinates can be used.
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you right click with the mouse. If you have a macbook, put two fingers on the touchpad and click once.
Once you have installed Too Many Items, when you bring your inventory up, you can choose from a list of blocks/items that appears by the side and click them into your inventory.
Once.
to be able to build a mansion in minecraft you need to be able to imagine what you want to build and you can get the blocks once you are in creative mod or if you want to challenge yourself you can build it in survival or in hardcore
right click the stack and add it will give you half if you want to split it more then right click it more the once.