The Africanized bees swarmed from a lab somewhere in Brazil and gradually made their way to the U.S. They were not deliberately introduced and it would be nearly impossible to eliminate them since they can mate with other bees. Africanized bees produce more honey, but are more difficult to manage. Beekeepers will introduce European queen bees to Africanized hives to try to dilute the African traits and diminish their aggressiveness. They are still much more dangerous to deal with than a docile queen.
Some queen rearing operations have very aggressive Carniolin bees because they are almost constantly kept queenless. Queenless hives get very cranky. More aggressive hives are stronger, but most people, especially beekeepers in urban areas prefer hives that are less aggressive so they are easier to work and won't attack the neighbors. When hives become too aggressive the aggressive queen is replaced with a more docile queen. If the hive is very docile, it does not always defend itself as well from more aggressive bees in robbing season and they are more likely to starve.
For most hives, the guard bees will defend the hive from anything within 25-35 ft diameter of the hive. They do not like strong smells and fuzzy clothing in dark colors. They defend the queen, brood, and honey = home from anything that may want to harm them. Swarms have an old queen, but no brood or honey to protect. They only surround and protect the queen. Swarms are usually not aggressive and if a beekeeper is called immediately they can remove the bees safely from an accessible area. If it is not accessible, they cannot do it safely. Bee swarms move short distances to rest while scouts look for a permanent home. The queen has been starved and she is not used to flying so she needs to stop more frequently to rest. Once the scouts find a suitable home, they will try to convince the hive that they have found the ideal home and if the queen approves of the accommodations they move in and set up housekeeping. Moving bees once they have decided on a permanent home is harder to do.
Foraging bees are more interested in gathering honey and nectar than in attacking anything. If you pay attention, they will almost always give a warning buzz to tell you to back off. If you start screaming, waving your hands and swatting them, they will think you are attacking them and defend themselves.
I have bees in my yard and in the herb garden everyday. Not at much as before 2011 when the varoa and hive mites arrived and nearly wiped out every bee. Bees usually forage after it warms up in the morning and again in the evening. On mild days, or after the sun comes out after it has rained a few days, the bees might be out all day to catch up. I can usually get within a foot of the foragers and I usually wait until they have moved on from the area I am working or I use the water from my fan spray to nudge them over to another part of the garden. I think the bees are used to me working in the garden and know I am not out there to harm them. So, as long as I pay attention when they warn me to back off and I don't accidentally grab a bee, we share the garden.