Villages are an important source of resources in Minecraft speedrunning. They can contain haybales for food and easy access to items such as beds, wood, stone, iron and obsidian for later progression in the game. The village type depends on which biome you are in, and there are 5 of them with some similarities but also some important differences.
Villages can be quite large, so going through every house can be quite slow, especially since some houses aren't useful. Generally the main resources you will want to get from a village is food, some beds and a bucket.The most valuable house in every village is the weaponsmith which is mainly called the blacksmith for historical reasons since its chest has the most valuable loot, can contain iron pickaxes, iron ingots, gold ingots and iron armour and it is marked by a grindstone. Other notable houses are fishermen houses which can have buckets in their chests, marked by buckets.
The main way of obtaining a bucket is killing the iron golem in a village. By hitting the golem and pillaring up three blocks you can easily kill the golem without much risk. However you should be careful as iron golems do a lot of damage and have a large range. Iron golems drop three to five iron, enough to craft a bucket. If you only get 3 it may be awkward to obtain a flint and steel though.
Savanna Villages spawn in the savanna biome and have haybales which are very useful since they can be crafted into wheat and then into bread for food. Crafting a hoe can be useful to mine the haybales faster. The blacksmith in this village is quite large and rare. The most notable difference in this village is that normal house chests can contain buckets so it can be worth it to check them.
Plains Villages spawn in the plains biome and have haybales. The blacksmith is quite large containing two lava sources.
Desert Villages spawn in the desert biome and have haybales. The blacksmith is quite large containing two lava sources. Make sure to obtain wood logs before heading into a desert since there are no trees in the desert. Otherwise you will need to break dead bushes for sticks and use crafting tables inside houses.
Taiga Villages spawn in taiga biomes and they don't have haybales. One big advantage of these villages is their chests contain logs, however food is awkward to come by and you can find bread in chests, or trade emeralds in chest with a farmer villager for bread or potentially cook potatoes from chests on campfires in the village. Also there is some haybales underneath some campfires. Also the blacksmiths are more difficult to identify since they are smaller but they are more common. Another note is that iron golems sometimes do not spawn in these villages, so it can be more awkward to obtain some iron.
Snowy Villages spawn in snowy tundra biomes and they don't have haybales. One advantage of these villages is they tend to have lots of furnaces which could be used for smelting food/iron. Other than that, these villages tend to be not that useful.
Once you've finished looting a village, you should be ready to build a Nether Portal to enter the nether.