I'm new to the YouTube channel game. If people set up multiple channels, do they set them under the one username account (if this is possible) or on separate username accounts? Are there pros and cons? And for the purposes of monetising later down the track, are the required counts to get into the program just related to the individual channel or can they be the combined views of all the channels under the one account?
You can do them separate, but it may better to have them under your main account. I believe the limit is 4, but I could be wrong. I had messed up my main channel long ago by posting different types of videos under the same account, so if you are doing something different, definitely do another channel.
For general intents and purposes, those channels you create under one will be treated separately . No one will know they are from the same person unless you make it known. There are, however, benefits to making them under your main account. Monetization requirements are also separate, but the financial information from Google is carried over. The biggest benefit to me is being able to quickly switch between channels.
There is a negative, but it's more something you just have to keep in mind. If you have a separate Google account to manage your new channel (like I do with this one), you have to remember that your new channel is setup under your MAIN Google account. If your main channel is AAA and you setup a sub channel BBB, when you sign up for services like Tubebuddy, you need to make sure you select AAA. From there you can select which YouTube channel you want, BBB. I had that problem again recently when setting up Patreon where all my social media accounts would work except when I tried YouTube. Would get the pop-up asking which account it was for, I would select BBB. Pop-up would disappear, but YouTube link would remain blank and no error given. Was frustrating for a while, and whilst in the middle of writing them an email to find out what the problem was, I remembered and selected AAA, then it prompted to where I could select channel BBB.
Can be a little confusing at first, just try to remember that "account" and "channel" are different at that the "channel" you want to sign things up for is a child of the "account" it was created under.
In short, it would come down to preference. For financial ease and navigation, it's better to make them under one account. If you want to keep things totally separate, just make a new account.