What is p=quarantine?
This is part of the DMARC record. The p= value is used to tell the email server what to do with email that fails the authentication check. Quarantine can be sending to the spam folder, or it can be sent to an email network quarantine holding box. In this help article you can learn more about DMARC policies.

How do you know customers aren't engaging if you have large email list?
There is the email health widget that has some stats that will help you message your mail practices one of them being around engagement. There are some other free online tools e.g. Smart Score by EmailSmart that can help find your email score.

What is low engagement?
This is when the users stop opening or clicking with your email over several emails over a certain time frame. You can check engagement in the email health widget or can use free online tools e.g. Smart Score by EmailSmart that can help find your email score. Here is a useful help article to learn about ‘Engagement Days’ metric. This metric measures the contacts that you have sent email marketing to in the last 30 days and the average number of days since they have last engaged.

How do I track engagement if I am sending from a subdomain and replies are going to another inbox?
If the email is being sent from Keap, you will still have access to the engagement data whether using a sub-domain or not.

How do email providers differentiate between an engaged vs. unengaged email?
Email providers watch how recipients interact with the email you send. If the recipient highly engages with an email, email providers identify who sent it and ensure it hits the inbox. On the other hand, any mail that the recipient does not engage with, open and quickly deletes, or just deletes without opening, that mail will be seen as a pattern that the algorithms will then send right to the spam/junk folder.