I want to create an animation using Blender for the model. Typically, when making an animation in Blender, the final output is a video file. However, I'm wondering if it's possible to save the animation data itself directly onto the model. In doing so, I could then use a different viewer, such as Three.JS, SwiftUI app or another compatible tool, to play the animation on the model. Is this feasible?
Yes, you can save the blender file and it will save the animation, so then anyone you share the blender file with will be able to see the animation.
Any program or website that can read Blender files (Like Unity and Sketchfab) will be able to read some of the animation as well. You can also export to FBX or gltf, which will preserve some of the animation data. However, the export process primarily preserves "skeletal animation" which is animation on "armature" objects in Blender. You can animate in other ways in Blender (like shape-keys and modifiers) but this animation may not export well to FBX or gltf or import well to other programs. So, if you are planning to use the animations you make in Blender as animations in other software, plan to animate using armatures, and do small tests to make sure your work will transfer properly.
For Three.JS specifically, the gltf file format is probably best to transfer animation data.