These are all free for you to use as templates for your own mods. This includes characters, stages, buddies, and every skin template for the base game cast.

Once you've downloaded them, you can find the files to copy by going into your file manager and going to "C:\Program Files (x86)\Steam\steamapps\workshop\content\383980" (assuming the default Steam location). From there, you select the folder that corresponds to the numbers at the end of that character's workshop link (ex. Sandbert with Phone's link is https://steamcommunity.com/sharedfiles/filedetails/?id=2280416081, so if you wanted to use it you would need to find the folder labeled "2280416081".) Copy the folder and all files to "C:\Users\{Your Account}\AppData\Local\RivalsofAether\workshop", and they will appear in-game!

More information can be found at the official guide below.
https://rivalsofaether.com/workshop/