1. having one password for all accounts is an easy way to remember passwords, but it will expose you to what risk?

But the risk is that if a hacker guesses that one password, he will now your password to all the other websites.

If someone was to find out one of your passwords then they can easily guess the other ones because of that one they found out.

So if your Roblox game account passcode was ChrisPBacon and I guessed it and so was your gmail account I can go on your gmail and say "hmm, maybe it's the same as his roblox account code then I can gain access to all your private info leading to me if I was a bad person taking your id and private info.

I have all my passcodes diffrent personally. Or you can put 2-step on for extra security. 

I hope I helped you lots!! :3