Required:
Items & Blocks.
Building A Structure In Depth.
Structure Types.
Optional:
Terrain Blending.
How To Use WorldPrimer.

You might have run into an issue trying to create a spawn structure in finding that the player doesn't actually spawn inside the structure. This is because Minecraft's world spawn isn't a fixed position, but rather an area. To place players at a precise location in your structure will require additional mods that can run commands when a new world is created. Now while this allows you to set an exact coord for your world spawn, you'll still have to contend with varying biomes heights potentially causing the player to spawn on top of the structure instead of inside. If you encounter this issue, you'll need another mod to modify which biomes are valid as world spawn points to force flatter biomes such as plains. These methods work regardless of whether you're using an exact position in your static structure or a relative position. The former simply has two additional commands that you'll need to run.
Note: To make sure the player actually spawns inside your structure, make sure there is enough open space inside at the position you're setting as the spawn point.

Setting up the commands:

Install the mod WorldPrimer, open its config, and at the very end enable "true" for the following toggles: enablePostWorldCreationCommands. Then scroll up to this section and enter the following list of commands into the field:

S:postWorldCreationCommands <
	gamerule spawnRadius 0
     >

If using exact position and not a relative position, you'll also want to add these commands to match the coordinate you set for the structure: setworldspawn X ~ Z and tp @p X ~ Z. This will force the spawn inside your structure. Leave the Y level blank so you don't accidentally suffocate your players it the spawn biome happens to be hilly.

Fixing spawn biomes:

If you're having issues with bad spawn biomes messing up players appearing inside the spawn structure, then you'll have to force a flatter more predictable biome. Install BiomeTweaker and its core mod, and then go to the folder config/biometweaker/scripts/ and add a file named scriptName.cfg. Name it what you want, but it must be a .cfg file. Then add the following script to this file:

spawnRemover = forAllBiomes("namespace:biomeName")
spawnRemover.set("isSpawnBiome", "false")

"Namespace" and "biomeName" would be the mod and biome ID. E.g for vanilla biomes; minecraft:swamp. Make a comma separated entry for each biome causing you issues. Note the format must have the comma after the quotes like this "biomeID","biomeID","biomeID"....

Alternatively you can set an exact spawn point with a defined Y height if your spawn structure doesn't use a surface placer and has a specified generation height (e.g floating, underground, specifically at Y60, etc), as it will always generate at that exact position so you don't have to worry about accidentally suffocating your players.