The misleading part about almost any mention of "millions of degrees" is that people implicitly expect this happens under earthly conditions, when in fact these values are typically just a reference to the speed of atoms in a rarified environment. Like the sun's corona, it may be very hot, but not very dense. In other words, there is not a lot of energy per volume, just a lot of energy per particle.

For example, a common way to reach "millions of degrees" is to pump the air out of a container to an extreme degree, then introduce trace amounts of a gas, and then accelerate or otherwise heat those few particles.

If this happened under normal pressures, there would be no container material that could withstand these temperatures. But in a near-vacuum, the collisions between the accelerated particles and the walls can be kept low. In an experimental fusion setting like this, they also employ a magnetic field to keep the plasma away from the container.

I'm not sure about the garage, but you can sure do it in your microwave - NileRed did a video about it: https://www.youtube.com/watch?v=l0u8Vtf2GoQ

Aaand he had to break all of his beakers afterwards: https://www.youtube.com/watch?v=tGqVMbAQhBs

The plasma you can generate in your microwave is nowhere near 100M degrees. The low range of plasma temperature is at around ~6K Celsius.

Such amount of plasma at 100M°K will destroy the whole building. Temperature of nuclear explosion is less than 1M°K.

Sure, here's a guide from Make Magazine:

https://makezine.com/projects/make-36-boards/nuclear-fusor/

Build a fusor running roughly at 10kV (Microwave transformers can do that if you modify them a bit, otherwise starter transformers from light fixtures or your car may be able to handle it).

That'll get you in the temperature range. You can also up the voltage to get more exciting effects, fusion is fairly easy to achieve here. Just not net positive fusion.

At the high end what you even mean with temperature gets a bit weird, everything is mostly a very rarefied gas.

That's a really silly statement.

I cant create 100 tonnes of nuclear waste in my garage either. However, it's not means for celebration when a giant institution does it.

As OP pointed out, with fusion -- net positive power output is important. Not the temperature achieved.