In general, you can't really re-use a rig made for another character. Unless that character is exactly the same proportions, and same deformation features, it will be a lot of unnecessary work to make them "fit".

That being said, lots of rigs are not made from scratch, but instead generated with addons. Rain and Snow are made using Cloudrig, which is a custom Rigify set.

So if you want to generate a rig like these, you just have to download and install Cloudrig into your Rigify and use it. It's all explained in Cloudrig's documentation. Then it's basically like generating any Rigify rig.

Before the technicalities, you have to consider the legalities. Can you legally reuse a rig (or anything) that isn't yours?

Most laws consider that you cannot do this unless the rights holders explicitly state otherwise. Which means you have to check every time.

In your specific case, Rain and Snow are both published under CC BY 4.0, so you could use them with conditions.

For the technicalities: in general, you can't really re-use a rig made for another character. Unless that character is exactly the same proportions, and same deformation features, it will be a lot of unnecessary work to make them "fit".

That being said, lots of rigs are not made from scratch, but instead generated with addons. Rain and Snow are made using Cloudrig, which is a custom Rigify set.

So if you want to generate a rig like these, you just have to download and install Cloudrig into your Rigify and use it. It's all explained in Cloudrig's documentation. Then it's basically like generating any Rigify rig.