When ever my PCs go to a new town, city, etc. I never know how to construct it. How many people live in it? Whats in the market? What makes the town unlike every other one they have been to? I want to know how you experienced DMs make your villages and towns. Tell me as much as you want, all advice is helpful!

There's a game by Vincent Baker called Dogs in the Vineyard that offers a really excellent method for creating towns. I'll paraphrase it down to this: focus mainly on what is wrong in the town. Something has to be wrong, otherwise, why be there? When the players arrive, there needs to be someone acting strange, something bad that recently happened, or things are plainly just not how they are normally supposed to be. So I start by asking, and then answering "what's the problem here?"  After you've considered the question, ask "what is going to happen because of it?" Is the problem going to lead to plague, bandit attacks, or government corruption? Next, "Who's in charge?" Obviously whoever's in charge knows about the problem, are they creating it, opposing it, or neglecting it? 

I personally find things like population, market economy, and what buildings are in the town to be tertiary information, that only matter if your players are interested in it for their own reason. The problematic events going on in town, and who the characters are that live there are what the players mostly interact with, so my advice is to focus on those aspects foremost. 

Why are they in this town?  Failing that, what do I want to throw at them?  That's how I figure out distinguishing characteristics.  Everything is based on what I want the PCs to encounter/have access to/have to deal with. 

I start with a basic idea of a towns purpose. Farming, mining, industry, historical significance, etc... then write a short backstory. I tend to default available supplies to what's in the PHB unless there's something specific I want available to the players. 

For example, in my last campaign I created a town called Orc Town. A large mining community that was made up of mostly half-orcs, halflings, and dwarves. After deciding that I populated it with a few specific shops and characters. 

To make the town stand out, the whole town was built over another underground town (the deep district). And being a town of miners I decided they were tough, so everyone in town is always armed. Mostly for show.

And lastly I made the town racist against humans. "The only good humans a dead human." To add a little more flavor I created an inn, the Dead Mans Inn run by Mr. Body, an undead human. (The only good human, get it?)

Just remember that if you put the Addams family in the town you will most likely think up quite a few adventure hooks, downside is all other towns will be rated against that town.

I create the world and i dont care about characters. The world would continu spinning if the adventurer werent there. Then once im done... I add whatever main quest to that city. Usually the players dont expect you to create a city for them. They expect to visit a city they have not explored yet. They kust know a city lives without them.

All that said the dmg as a ton of info on city building including tables for everything you actually asked ! Which theyd make something out of those tables.

I feel like it's always really helpful to have a map of the city to use as an overview.