I recently completed the whole process and it went super smoothly. Here is my experience:
My plan was to land, quarantine and come back to the US. Flew to Toronto from US on 18th July. The officer was very nice and polite. Asked for my passport and COPR. I hardly waited for 5 minutes while this process completed. Since my case was based on spousal sponsorship, one member here warned me not to travel alone and have my spouse travel with me from the US (which I felt was unnecessary burden so I decided not to risk the health of my wife and had her stay in US as I travelled by myself). After he stamped my COPR, I went to the next desk where I was asked questions about my health, my temperature was recorded and quarantine plans were fed into an iPad. That also went extremely well. The whole process took less than 30 mins (not very many immigrants were present there).
After quarantining for 14 days, I travelled back to the US today. Immigration happened at the Toronto airport and the CBP officer was again very polite (which I found surprising). He didn't even ask for anything other than my passport with the valid H1B stamp. I was out and about in 5 mins.
I hope this will help. All the best to everyone who is travelling.