Question regarding the visa dates.

1 Situation, 3 dates:

I am a 51 y/o Canadian on a Non-Imm O-A.

My visa sticker is valid from Nov.20 2020 until Nov.19 2021.

My entry date stamp is 05 Dec 2020 and has the written date 03-12-2021 on the "Until" line.

I just did my 90 days report using the application and It says valid until Nov.19 2021.

Which is it? I can stay until Dec 03 or Nov 19?

Now, I will do a extension request. What is my best strategy?

Should I do it as soon as possible or wait as late as possible?

What will be the new date? Will it be an extension adding 12 months to my actual visa, taking me to Nov. 19 2022 ?

Or will it be a year from the date of extension approval ?