Vaginas contain a balance of bacteria and yeast, when there is an imbalance you can get a yeast infection (excessive growth of yeast). Some common causes are from hormone changes, birth control pills, antibiotics, douches, weak immune system, passing through sex, or eating lots of sugar.

To answer your other question there’s different types of yeast so it is not the same yeast in bread lol. The type of yeast in and on your body is called Candida and it is natural and beneficial to the body as long as it doesn’t over produce.

Some women are more prone to them than others. Yeast grows in moist environments so you can get them from staying in a wet bathing suit or sweaty workout clothes. Its recommended to keep that area clean and dry to avoid yeast infections. The vagina has a natural ph balance which allows it to "self clean" it keeps bacteria in check. Sometimes products such as soaps or lube can throw off the ph balance allowing for bacteria and fungus such as yeast to grow.