I'm currently in the middle of adding hood replacements in my game, including replacements for Downtown, Unis, and Vacation Destinations. In the descriptions of these replacements, they usually say that they are "empty" with "no sims/townies/downtownies/dormies/NPCs/Tourists/etc". Sometimes the creator provides links to a townie "pool" that can be downloaded separately so that you still get these sims included.

I was wondering, if I placed these empty hoods into my game and start playing in them, will townies/downtownies/dormies/NPCs/Tourists spawn on their own? Or will there just be no sims whatsoever in these hoods and it's up to me to create all of my own or download a townie pool?

That depends on how you set things up. Unless actively suppressed by a mod, townies and NPCs will spawn when needed. No townie respawn will suppress respawning of townies when old ones become playable, but will not suppress the spawning of an NPC for a coffee stand or of a social group townie to inhabit apartments where a playable lives. Have a clear idea in your head about what sims you wish to make yourself and which you’re willing to have the game spawn for you, and look into the combination of mods and empty templates that will accomplish that.

Townies spawn from templates in your install. If you want none to spawn you need to deal with those templates. NPC's like the hotel maid spawn as needed. I did once make it so no locals or townies spawned and that was weird so I had to make a few. Made them in CAS then used the townie brick.

There's a Mod on here that stops the game from spawning NPC neighbors in apartment lots which I've had in my game for years and I still get townie neighbors if I create some in CAS and move them into spare apartmets to provide townie neighobrs for my main residents.

Basically, yes! If you install an empty neighborhood, townies and NPCs will still be generated, and they will continue to do until all the needs are met. For example, if you go in a dorm with 50 available rooms, 50 random townies will be generated for your university, and as you continue to play, all the NPCs required for university will begin to spawn; cafeteria worker, cheerleaders, mascots, streakers, etc... If you visit a lot where there's a restaurant, the host, waiters and cooks will be genetared. Basically the game won't let you play without these needed characters.

If what you want is to keep the game from generating them, then you need mods to do it. These are the ones that I use:
No Townie Regen stops the game from generating townies randomly.
No Stray Respawns stops the game from generating stray cats and dogs.
No Dormie Regen stops the game from generating dormies to fill all the rooms in a dorm. The rooms will stay empty.
No Townie Neighbours does the same thing as with dorms, but with apartments. The game won't insist on filling every apartment unit with random neighbors, so they won't be spawned.

There is no way to stop NPCs from generating because you need them for the services in the game, but there are ways around it as to avoid using those services, or to use Anti-Redundancy so the game only generates 1 NPC of each kind, or There Can Be Only One (Professor) for universities to generate only 1 professor per major instead of 2. There's also No more newspaper deliveries, No Greeting NPCs (Garden Club and Matchmaker), No Rod Humble, etc...

Long story short, it's a lot harder to keep townies and NPCs from spawning, so if you do want them, you don't have to do anything and they will show up on their own.

@LauraPamplonaS -I've got most of those though I've got a Mod that lets me choose if Rod Humble is going to show up and that no townie neighbors stops the game from generating NPC's as townnie neighbors though I can still have some townie neighbors in my empty aprtments by going into CAS to create randomized sims and couples as townies to move into an empty apartment and have playable townies.These are families created to flll out the town's population.

Thanks so much everyone for your responses! I'm glad to hear that the game will spawn sims in clean hoods because I am way too lazy to create all my own townies, haha. I may end up using those no-spawn mods if I ever decide to create my own custom hood with all my own sims.

There is an object that looks like a volleyball and will spawn townies for you. I can't remember its name or find it in a search of downloads, but I'm sure someone will know what I'm talking about. I have used it to make townies and downtownies for my clean hoods, and I think it does a good job of making an interesting variety of Sims. I've even moved a couple in and started playing them. It's a good tool for the lazy (which I am as well).

There's another thread where somebody linked to some townies if those are needed and I donwloaded the version that made them as bin sims to be used as playable townies for apartment neighbors and I can also create my own townies in CAS.