Edit character mesh while in runtime

I am new to the forum so I tried to put it in the place where it best fit, I don't know if this is the right place but I'll post it here anyway.

Some background information, I consider myself not an advanced coder/game developer but an intermediate.
I can mostly make most of the stuff myself but a part of the stuff I have to search it up in youtube videos or posts.

Ok, back to the issue. I want to make a game where you can literally fully customize your character, you build everything. Of course, it would take a lot of time and practice for a player to build their character, so I want to make some tools to facilitate the process, some premade prefabs that you can just drag and drop and somewhat edit.
If you have played Spore before, it is literally like its creature creation. I want to remake that. You can choose from a variety of arms, place them and modify them to make them bigger, smaller, larger, shorter. Same with the legs, body, eyes, etc...
This will be an alien game so it's ok for the player you make to look weird, its main purpose is also to make funny looking characters.

Here are my two issues/questions
1-. How can I edit meshes in realtime to make a drag and attach to the main body and have the meshes run smoothly through and the coloring does the same. And also be able to edit the mesh to make it bigger on certain joints or smaller, larger or shorter, etc, just like the spore creature creation.

2-. Somehow, make walking, running, idle, etc, kind of animations with these custom characters. It would be impossible to rig it and animate every single combination as there is no limit whatsoever to the number of changes that can be done to the character. I know this is possible because, once again, spore creature creation has done it before.

So in conclusion, it would literally be a more compact and simple version of blender added into the game.
I hope someone out there knows how to do this, I've looked around a ton and can't find anything related to what I wish to do.
Once again, I am an intermediate coder/game developer, so I don't understand everything.