> plasma means that fusion is actually occuring, rigth?
As mentioned plasma is just another state of matter[1], where a significant portion of the electrons and ions a separate rather than combined as atoms.
Fusion happens when you overcome the electrostatic repulsion of nuclei, bringing them close enough together so they can fuse[2]. Typically, in reactors like this, that means you confine (compress) a sufficient amount of material ("fuel") to a small volume and heat it up sufficiently. Both are needed to make it possible for the nuclei to come close enough to fuse. The heat required is so great the material will turn into a plasma.
> And does anyone know how this one collects the heat and converts it into electricity or whatever?
This depends somewhat on reactor design, including fuel used. However they're all fancy steam generators in the end, so not unlike a traditional nuclear power plant in that regard.
From what I know, typically the "surplus heat" of a fusion reactor comes in the form of energetic neutron radiation[3]. This radiation is ionizing and as such shielding is required, and this shielding will heat up as it slows down those energetic neutrons.
In the ARC reactor[4] for example, a liquid shielding "blanket" surrounds the fusion chamber. As the neutrons heats up the liquid, the liquid gets pumped through a heat exchanger to produce steam to run a steam turbine.
edit: I found this talk[5] from one of the folks behind ARC to be very illuminating in how fusion power works and the challenges involved. It's from 2017, but the basics haven't changed.
[1]: https://en.wikipedia.org/wiki/Plasma_(physics)
[2]: https://en.wikipedia.org/wiki/Nuclear_fusion#Requirements
[3]: https://en.wikipedia.org/wiki/Neutron_radiation