There are many varieties of edible mushrooms, all have slightly different requirements. Just like plants, there is no set of rules for all of them.
Yes, the easiest way is to buy grain spawn. I have taken the next step, and start my own cultures. It's not much harder than home brewing, but it does require patience and some knowledge. The only reason I taught myself how to make spawn, was because I couldn't buy any gourmet mushroom spawn in Australia. Now I can explore plant tissue culture!
Mushrooms do continue to grow from the substrate after the initial harvest.
I have kept paper spawn (Pleurotus eryngii) in a cupboard, unopened, for over a year, and as soon as I spawned it to new substrate, it started growing.
At $45/kg for gourmet mushrooms in the markets, that's not exactly futile. If you're talking about Field mushrooms (Agaricus), they're generally mass produced, and fairly cheap. The substrate for them is also a bit tricky to make on a small scale.
Sterillization is very important when cloning or starting cultures (just like plant tissue culture). For me this involves cleaning a small glove box, and Pressure Cooking a jar of agar and some jars/bags of grain. I don't wear any suits, just some gloves (sometimes). The total amount of work time is not that much, waiting for the PC to cook, then cool, takes a while.
If you start with colonized grain spawn, then you don't need to sterillize anything, although it can help sometimes. Most home growers use hot water, 60-70 degrees C for 1-2hours, to pasteurize wheat straw. This kills most competitors, but leaves some heat loving organisms like actinomyces alive, and they not only provide nutrition for your mycelium, they also prevent lipstick molds and other nasties from taking over.
I bury my spent blocks in the garden, and they continue to produce mushrooms subject to weather, i.e. after good rain.
Commercial growers know exactly the day their mushrooms will be ready, they control everything, and they have multi-million dollar setups. Home growers have been successfully growing mushrooms for thousands of years, with no sterillization or fancy climate control.