Question Any alternative to create game character by sprite sheet?

Hi, I'm new to Unity, the version of Unity I'm using is 2020.1.8f. My problem is that I just watched a official Unity video(Lost Crypt) about:

1. Importing an image created by Photoshop to be used as a character in the game.

I am a student and Photoshop is not affordable option to me. Regarding this, what other software would you recommend?​

2. After importing a multi-layers Photoshop image into Unity(by something called PSD-importer package), they added skeletons for the sprite(an assassin cat?), which is an astonishing feature to me that it is no need to have a sprite sheet to animate it.

Does this "adding skeleton" feature only work for multi-layer-Photoshoped images? So I cannot, say download a single-layer image online and do the same to animate it? More specifically, would this package/ the Unity engine provide way to automatically "cut the single layer into multi-layer image" for me? If not what software have this feature or this is not recommended, I have to draw it myself: head, body, legs, and so on?
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3. So other than the two methods above: Using sprite sheet, using sprite "skeleton". are there any other recommended way to create a sprite in Unity?

 

many people like asesprite for pixel art, alternatively you can use gimp or idraw

you can use skeleton with any sprite, doesnt matter if its from photoshop