Does Electrum store my seed in their server? I am new to Bitcoin. I used Electrum software to create my first wallet. When the software generated 12 words, I had written down that in paper. These word are very important, right? But the software can see these words and stores them somewhere in their server. So how do I know that they don't use my wallet because they have my seed? Sorry if it's a dumb question.
Electrum does not store your seed, the server is run by volunteers, and you can run your own if you wish.

You needn't take my word for it: Both Electrum and electrumx are open source, and you can verify the functionality, and build a version from code you have verified.