Also, what is going on when I click receive bitcoins and a random address pops up? Why isn't there only 1 address to receive bitcoins from? Is there a way to look at how many btc I have on BitcoinCore? Thanks.
It is recommended for privacy reasons to generate new addresses for each new transaction. Otherwise it would be very easy to track your payment history. If you want to use the same one over and over that is fine too, but now you know why to avoid it.
You can check the balance of the address you sent your BTC to with a block explorer.