For questions on Google Multiple Accounts Login, which allows a user to be signed in to multiple Google accounts at once. (Questions about accounts on smartphones are off-topic here.)
Originally introduced in 2010, the Google Multiple Account Login is a tool available in most Google products that allow the user to be signed in to multiple accounts simultaneously, and allows for switching among any of the signed in accounts with a universal drop-down menu.
Logging in to a different account is as simple as clicking your profile photo in the top right and clicking "Add Account" (to log in to another account) or clicking on the account where you're already logged in. If logging in for the first time, you'll be prompted for username, password and (if applicable) second factor authentication. From that point forward (unless you clear the history/cookies in your browser) you'll remain logged in to both accounts and can easily switch between them. Adding a third (or more) account is the same process.
The "default" account is simply the account that you logged into first, and if you go to a Google product without switching that is the account that will log in.
Not all Google products support Multiple Accounts Login.
Questions about Multiple Accounts login should be concerned with the web interface. Switching accounts on a smartphone is off-topic here, and should instead be asked at Android Enthusiasts, Think Different, or Windows Phone as appropriate.