I don't believe mushrooms are suited for natural fermentation process. They are usually pickled in vinegar, but this has nothing to do with fermentation. Mushrooms don't taste sweet and this means they don't contain sugar and without sugar food will not ferment. Cabbage, beets, fruits - all these foods are rich in sugar so they will ferment into sauerkraut, fermented beets or wine. Grains contain sugar and will ferment into beer.
Then, you need a supply of lactic acid producing bacteria which for example cabbage and beets contain. Fruits contain sugar and yeasts. Yeasts will break sugar into alcohol and
CO2 carbon dioxide gas (soda gas). In cabbage lactic acid bacteria will ferment sugar into lactic acid and CO2 carbon dioxide (soda gas). Even meat contain very little sugar (glycogen) and lactic acid bacteria and this is enough to make a traditional salami, which is, however, air dried and not fermented product. If you add more sugar into meat you will increase fermentation and more lactic acid will be produced. The result is sourly sausage like Summer sausage which many people seem to like.
To make it simple not all foods contain enough sugar and naturally present bacteria or yeast to start fermentation. You can add extra sugar and a fermenting starter culture, but the culture will ferment sugar and not the fruit. A lot of acid will be produced which may help to preserve food but that will be like immersing food in vinegar. Not fermenting it. Bacteria hate acidity so you can place meat, eggs, mushrooms and other items in vinegar and they will be preserved, however, the taste is acidic. Fermentation produces its own flavors.