I'm not 100% sure of what you are trying to achieve.
However, I'm going to assume you mean you want to import the FBX files, and have all the animations in your blend file with just one character.
>>if you want to import EACH file/have to check/clean up each animation<<
I'll use these two characters/animations from Mixamo as an example:
First, import your base FBX/default, whatever it is you want. To ensure the best import, make sure you uncheck "add leaf bones," (shown in image) under armature. You MAY need to check automatic bone rotation. FBX can be fiddly when it comes to importing, so I'll refer you to the documentation for extra info:
Cleaning up FBX models is not a task I'll be addressing. If you need help with that, I suggest YT.
My character is now imported, and the animation is in the timeline. If I press play, I can see the animation and test that it works.
Now that I have my first character in, I need to import the other, using the same settings as before. We want to make sure that everything is exact.
(I recommend making sure you organize things into a collection, too, so that you know what's what.)
Notice that the animations are, obviously, two differently lengths. I've played them, so I know they both work.
Now, on the right hand side, I went ahead and renamed my animations, just to keep things clean.
Go to the animation panel. Select the armature, then the animation that you want to copy. I recommend hiding the other body. (That was NOT a suspicious thing to say.)
On the left hand side, make sure you're in the action editor. Again, make sure the animation that you want to copy is selected. Press the copy button (two pages pic.) Name it whatever you want and make sure the shield buddy is on! (This ensures the animation saves.)
You should now have a new animation in which you can play in the og model, which you can now cycle through.
(Yes, you could have cycled through it before that, but this is so you can preserve the og animation.)
I'm going to go back to my original model/base model here and now we're going to make it so we have multiple animations.
I am going to hide the the second model as well, since I won't need it anymore.
See that button there on the bottom left? Select your animation, press "push down." Then, do the same thing for the second animation.
To read more about NLA/storing actions, see the manual here: https://docs.blender.org/manual/en/latest/editors/nla/tracks.html#action-stashing
It should look like this:
You can then go into the NLA panel, name stuff, whatever.
Uncheck or check whatever one you want to see.
Then, go to export to gltf. However...
Do keep in mind, as the image states that "When on, multiple actions become part of the same glTF animation if they're pushed onto NL tracks with the same name. When off, all currently assigned actions become on glTF animation." when that box is checked.
Unfortunately, I don't know anything about glTF. So, I'm afraid I don't know how to keep things really organized for you in that department or how that really works.
But if you import it again, you can test. I just went back they are there.
There are other methods for importing animations, such as importing and exporting BVH files, and if you search for these kinds of things, you will find various methods. But it basically boils down to the same kind of thing.
Hope that helps.