So I noticed all blocks in the game and even ladder have this design, so basically is this a thing which you can add or is it multiple blocks from each side.
Like there is not much lag in the game and even alot better from lag, if this not a part then what else can it be? please reply
Most likely this is done with multiple parts
if you want to use this part circled in red (Which is called a TrussPart)
in Roblox Studio you have to insert it using the insert object toolbar found in the model tab OR by pressing CTRL + I on your keyboard then typing in what you want to insert
It will not be laggy, Parts and TrussParts are built into the engine so you can have alot of them without affecting performance too much.
Using decals for an effect like that would be a waste of game memory.