Whats inside a Macbook Pro (13" 2015), which has a SSD, that makes it get hot?

A lot of the Macbooks have a very bad heat problem. Part of it is the fact that Apple likes to rely on the outer aluminum casing to radiate the heat into the room, that's why it is so quiet. It only starts up its fan during CPU intensive things such as gaming, video streaming and video editing/rendering.. but to answer your question, it's, for the most part, the CPU.

Heat comes from the processor that also holds the integrated graphics.
The most common reasons for fan noise and heat:
– Processor-intensive tasks, such as video editing.
– Video intensive tasks (HD video or games--HD video includes Youtube, etc.). Fix: Take a five minute break every 30 minutes to let the system cool.
– Using the computer on the bed. Fix: Take it off the bed.
– Case/cover attached. Fix: Remove the case/cover. This includes keyboard cover. The keyboard is the input channel for Air.
– SMC is corrupt. RESET SMC. (Link below)
– Hot room (in summer). Fix: turn on the A.C.
– Dust bunnies build up in the fan housing. Fix: Take the computer apart and clean the fan area. (Applies to older computers or those used in a very dusty climate)
– Using the computer on the bed. Fix: Take it off the bed.
– Fan is faulty. Fix: Take the computer apart and replace the fan. (Again, applies to old computers.)

@Mitchell: The aluminum will dissipate heat much faster than the plastic of a cheap PC case. Thicker PC cases have more air space inside and that can help move the heat out by fan. Some Alienware cases are TWO INCHES THICK. My god, why buy a notebook if it weighs TWELVE POUNDS. It ceases to be portable, more like a boat anchor.

<0-0>

CPU, motherboard, video card, SSD and power regulators.

A processor and graphics processor and assorted other electrical components packed in to a tight space.