[Google Pixel 2 Questions & Answers] djer34:
I would also like to welcome you to the community. Good choice crossing over from Apple and you made a great choice as far as phones go. Xda is a grea place to start and there are amazing people on here that can help you get to where you want to be.

Some background on myself, I have owned an android phone since 2010 and have rooted every android phone I've had since then. Root is the way to go and it opens so many doors for android devices.

1. Don't let a security risk scare you away from rooting, as long as you are smart with your device then you will be ok. You heard right in that Magisk keeps you safe when rooting. Magisk implements security protocols that allow our devices to pass safteynet checks. Safteynet is what android uses to tell apps that your device is secure and hasn't been tempered with. Things like Google Pay use safteynet checks in order to function. Some apps also won't function if it detects Magisk on your device. This is simply because the app knows your device is rooted and doesn't want to function as it knows it has been tampered with. Magisk has a feature called magiskhide that hides itself from certain apps being able to see it. So that, in a nutshell, is how Magisk tricks your device into thinking it hasn't been tampered with. Magisk also prevents apps from getting root acces to your phones that you don't want to authorize. Everytime an app initially requests root access, Magisk will prompt you with a popup asking if you would like to authorize said access.

2. As far as security goes. Google releases security patches once a month for our devices. if you run a custom ROM then most devs will incorporate these updates in to their ROMs and push them out to users. So when you root, your device will no longer update OTA, you must do it yourself. Even if you run stock Google firmware, you must still manually push updates to your phone (plenty of guides on how to do this). As long as you stay on the most recent Google security patch, then you have nothing to be afraid of.

3. Free apps. Yes, there are illegal markets out there that upload stolen apps but these come with a high security risk and are not allowed to be discussed on Xda. When you install apps from markets like these you run a high security risk. Some apps are made to look like other apps and have different permissions and can have access to parts of your phone that you don't know about. I would never recommend installing apps from markets like these. Stick to the Play Store and support the devs.

4. Like I said in #2, you will not get OTA updates anymore since you are rooted. Yes, in the past with other devices, some devs have come out with apps that allow OTA while maintaining root, as of right now there is not such thing for our phones. Manually pushing updates to our phones is not hard with TWRP. TWRP allows you to backup everything, wipe everything, and install new ROMs/updates. When you install a new ROM it will wipe everything as far as your os goes but will not wipe your storage (pics, music, files) So when you update a new ROM I highly recommend you get an app called Titanium Backup. TB will backup all your apps+data, that way when you install a new ROM/update, all you have to do is restore the apps with TB and its like nothing changed. It can even backup things like call history, texts, wallpaper selection, wifi passwords, etc. Definitely a must have for rooted devices.

Dive in, get involved, but please do your research before just spouting off questions in this q+a thread. People are here to help you but 9/10 times your question has probably already been answered. Read the rules of Xda and happy rooting.