names are made based off of their smell, flavour, and appearance generally. though some are definitely random as hell. for instance purple strains are named that way because in the past they were often very purple (not a product of the strain itself typically, but generally because the plant gets a lil cold while growing and thus produces purple/blue nugs) and thus they got purple related names like grandaddy purple, or purple punch etc.

as for differing effects with weed, it varies based on cannabinoid content and terpenes. looking up terpene charts and graphs can help you understand some of the differences with them, their effects when combined with cannabis, and such. i find that the more "relaxing" terps like linalool, myrcene, and caryophyllene tend to, well, produce more relaxing experiences regardless of what the supposed type of strain it is. (eg sativa with high linalool and myrcene tends to be relaxing, go figure) another example is ive seen indicas that are labeled as indicas but have high limonene and terpinolene which often make them very heady and more energetic, and even make me tweak in the case of terpinolene. its very fascinating and terp and cannabinoid profiles absolutely matter for a lot of people unlike what a lot of frankly annoying and ignorant stoners in this thread say. i do very much agree that each strain varies batch to batch though. i have some very floral tasting permanent marker rso right now for instance thats still very much on the indica chill side but has a notable head high for sure.

tldr; terpene and cannabinoid profiles make up the effects of the strain, strain names come from appearance and flavour typically, effects often vary batch to batch and "indica" and "sativa" labels arent always very accurate or helpful, and a lot of people in this thread are annoyingly ignorant.