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.
So I trust Electrum because it is run by volunteers and open source. But there are many others sofftware and apps in Google store claim that they are bitcoin wallet softwares. If I try using those other apps and provide my seed to those. How can I know that these apps won't store my seed somewhere else like Electrum?– DatNov 8, 2018 at 15:33
You don't, unless they are open source, and you have reviewed the source, and compiled the app yourself from source you had reviewed to verify the binary matches the one being offered on Google Play or the app store. Nov 8, 2018 at 15:40