To use animations from multiple assets, you need to follow these steps:

1. Import the files to Unity.
2. If you are using the Generic rig, you have to set the Avatar Definition to Create From This Model on your character (or reference character if you plan to have multiple characters with the same rig). For each imported animation asset, set the Avatar Definition to Copy From Other Avatar and choose the avatar of your character or reference character. To do so, you need to unfold the character asset, find the avatar, and drag and drop it to the Source field in the Inspector tab.

3. If you are using a Humanoid rig, you don't have to copy the Avatar Definition (you still can if your rigs are exactly the same; that way you will have fewer avatars in your project, making it easier to find the ones you need).
4. Create or open an Animator Controller.
5. Assign animations from different files and build transitions between them.
6. Run the game to see the animations work.