I wonder how easy it is to for a new employer to get info about me.

Unless someone really wants to dig up some dirt on you, only HR is interested on your past during the hiring process. No one has the time to do that. Employment verification stops at years spent there, job title, who was your manager, at least coworker to confirm you spent the time working on what you said you did. They don’t ask whether you were a good person or not. They don’t ask questions beyond that. Even if some volunteers some damaging information they don’t take it seriously because people have grudges. Your moral character is determined by your criminal background check. Usually, if ever convicted of domestic violence, employers would avoid you because they don’t know if you’ll ever snap at your coworkers. The assumption is, you’re not a team player.
In US, depending on which state you live in, most of what is available online is about your private life. Home address, names of your relatives, criminal records, if you’ve ever file for bankruptcy, how many properties you own.
If someone wants to find info on you, all they need is your phone. Plenty of websites out there will give them even email addresses of your relatives and people you know. Professional website like linkedin can give them the name of previous employer.
If someone asks for your phone#, give them a social media hander like your SW username 😉😂

Depends on how much chaos you created in the past. Police reports? Facebook posts. Pictures or video with circles and arrows. Time stamps

What did you do that you don’t want them finding out about?

many employers WILL search all social websites to see what you post!!

Extremely easy. However, look at everything thing they have on your background check. So many times it’s not correct info.

If you have been salacious in your social media, take it down for a while.

It depends on which country you are in. Here in the UK and all you will get now if you request a reference is confirmation of the dates you were employed, not a reference as such.

i don't think they keep any notes on previous employees

& If they did for some reason i believe any new management would not take the time to read them or care about it

i assume they do my company got taken over by MGM so they can do back round checks they may or may not

it’s my understanding that former employers will confirm the dates of employment.