Yes it will. There's no way to bypass it, if there is, that would be a serious security flaw - the kind that would get patched very quickly. There have been some phones which had a vulnerable bootloader that allowed this in the past (eg: OnePlus devices), but there's no such exploit available for current generation devices
I'd like to see some sources backing up your claim, which is applicable to current generation phones.
And rooting the phone requires an unlocked bootloader, which would present a warning when the phone is booted up.
eBay is fine. Just check the seller's reputation first before buying.
If overall positive feedback is greater than 99%, and they've sold hundreds of items, then there's no problem buying from them.
I've bought dozens of phones from eBay over the last two decades, never had any issues.
By checking whether the bootloader is unlocked or not. If the bootloader is unlocked, then all bets are off, but if the bootloader is locked, you can only run the original, unmodified firmware. Any alternation to the firmware will result in the dm-verity check failing, causing the system to not boot at all. The only data which can be altered is user data, which is wiped in a factory reset. So a factory ressr definitely gets rid of it.
If you're paranoid though, you could always flash the stock firmware downloaded directly from the manufacturer's website, which will override all system partitions, so you can be absolutely sure there's no spyware - besides the spyware included by Google etc of course, or the spyware embedded into the hardware by the chip manufacturers...
Support open source projects, such as Wine, KDE, Wayland, LibreOffice etc.
They're called Kaomoji btw.
https://www.unicodepedia.com/kaomoji/