Under change, there is the word used and a lot of btc addresses. The first btc address is the one i use a good amount. I shows a good amount of Tx there meaning lot of trasnactions. My balance in that address isn' that much compared to the other btc addresses i have below. Biascally i have around 10 btc addresses in that Used thing... where many have btc in it... which all add up to my btc balance.
My question is why are my btc spread out through all these addresses when many of these addresses i never used as a receiving address?
Electrum uses change addresses by default and any change left over after spending some bitcoins is sent to a new address under the change section.
The other thing is when i send btc to someone, which address is it coming from and shows to the other person/site? Does it show the btc address where i do the most transactions? Or does it come from the other addresses?
It will come from a random address or addresses unless you manually choose to spend from specific addresses or coin inputs using the Addresses/Coins tabs. You can right click on those and choose Spend From.
Also is it true the address that you use to send btc, they could see how many btc you have in that address? If so, that surprises me because wouldn't that present a security issue?
Yes. The balances of all bitcoin addresses are public knowledge. This is part of the reason that change addresses are used as they help increase your privacy by sending change coins to a new address each time and spreading them out over multiple addresses.
What if you use another wallet like blockchain or other wallet like armory etc. Does btc show up in different addresses even though you never used that address? Because it seem like when i receive btc, it would be broken up and sent to many different address in many of the Used addresses.
Different wallets handle change differently and most allow you to choose how you want to handle it. New addresses for change are recommended for privacy reasons but you could always choose not to use them if you didn't want to. They only get sent to the change addresses when you sent coins not when you receive them.