Stratification is the process of giving your seeds a period of 'cold' to prepare them for better germination. If you dont stratify seeds, they can still germinate but the rate is often much lower and they can actually take 2 or 3 times longer to germinate. A refrigerator is all you need. Most seeds don't need to be stratified. Generally speaking many perennials, that are native to areas with a freezing winter, benefit from a chilling period. I prepared a list of 35 plants that can benefit from being stratified. These are plants I often seed start.
There are two ways to stratify seeds. One is called wet and the other is dry. Dry is the only process I do because it is what is most practical when starting seeds indoors. Wet stratification is no more than planting your seeds in a moistened (wet) starting mix and exposing the whole set up to a cold period. That is perfect for greenhouses, cold frames, and places where you can manage temperatures. You can also place the seeds in a bag on a damp paper towel. I find the dry method to be effective with less work. Treat the wet method bag like a seed pack.
Seed packs can be placed in the refrigerator for 4-8 weeks. There is no information on exactly how long different plant varieties need the cold periods to last for their seeds. The above range seems to be fine. The temperature should be near 32 Fahrenheit or 0 Celsius in theory but most refrigerators are several degrees above freezing and that works. Buy your seeds, that need stratification, several months early and toss the seed packs in the refrigerator. Take them out about 5 days before planting and let them feel the warmth of your home.
Stratification can be done on all seeds, in theory. So when in doubt stratify them in a refrigerator. This helps address the problem of poor germination or waiting for germination. Fungus gnats are the other major problem when starting plants indoors because you are inside, in a small space, and the fungus gnats only have one place to go. They will infest your seed starts. They have no where to go, they have no enemies, and the water and warm home temperatures are all they need to hatch and multiply.
I recommend creating a sterile starting mix for that above reason. Microbes are not needed for seed starting and can be added later at potting up or when put outside. Here my the video that shows you how to use boiling water on starting mixes. Most seed starting mixes are called dry mixes and need hydration. Simply saturate your seed starting mix with boiling water and not tap water.
It's true boiling water will kill microbes, in most cases, but is well worth it to make sure you don't get fungus gnats. Outside, in large green houses, in cold frames and other large outdoor spaces, boiling water is not needed as fungus gnats appear but they seem to be less problematic. I suggest boiling water for indoor seed starting, in small spaces.
The best way to address the problem of fungus gnats is to do everything you can to prevent them. I have not found a peat based seed starting mix that states 'no fungus gnat eggs'. The eggs naturally end up in the peat moss that is used to blend most starting mixes. I am not saying they are in all starting mixes but I have used many brands over 20 years. If I am not diligent about prevention, they tend to show up. I will be doing a complete seed starting series for 2022 if you want to follow me YouTube.