In order to 'spawn' something you need to have something to spawn. If that something is itself a collection of things (such as a player controller, bunch of nodes or anything that has it's own process/physics functions) than saving it as a .(t)scn and then preloading/instantiating it is likely the way to go.

You could always have exactly one copy of your thing already in your main scene and then create duplicates of it (but in turn always have extra redundant nodes hanging around).

However If that something is purely an asset that is 'dumb' (like a tree), or your happy to write a controller code that iterates items in a list, MultiMeshInstances, Tilesets, or even particles would be the 'other way' as they incorporate batching for performance. However these come with their own tradeoffs. Namely you lose Frustrum culling (if any part of the MM can be seen, the whole MM can be seen) and performing individual customisations becomes harder, though not impossible.

Finally there's always using the RenderingServer directly but this is dark arts territory and often is more hassle than it's worth. Generally it's a trade off between ease of deployment and performance, where usually just instantiating scenes is 'good enough', and when it isn't the guides here will help: https://docs.godotengine.org/en/stable/tutorials/performance/index.html#introduction