Hello @Munshi - thank you for your question(s) and welcome to our community!
1. Can I go to Canada to work? (My aunt will provide the documents and contract)
Along with your Aufenthaltserlaubnis you received a Blue Passport that allows you to travel to every country that signed the Geneva Refugee Convention (Canada did so).
See these threads here on Wefugees to learn more about the Blue Passport:
As you can read there you are not allowed to travel outside of the European Union without a visa. In order to work abroad you need a work permission.
If you want to work in Canada you need to apply for a work visa. Please check out this website for extensive information: https://www.canada.ca/en/immigration-refugees-citizenship/services/work-canada/permit.html
2. + 3. Can I come back to Germany later? If I work there for more than 3/4 years, how the German will react to that?
Yes, you can. You will have to leave Canada anyway as soon as your visa expires (if you don’t/can’t prolong it).
If your stay lasts longer than six months I would recommend you to get in contact with Ausländerbehörde to ask them for their agreement. Otherwise it could happen that you lose your Aufenthaltsgestattung.
Source (German language): http://www.eu-info.de/leben-wohnen-eu/aufenthaltsrecht-eu/7400/
4. Can I get the citizenship later in Germany?
Yes, you can apply for it under certain circumstances.
Please read the information provided in this thread about getting the German citizenship: https://www.wefugees.de/k/questions/1755
5. Do I have to pay taxes on my earning in Canada?
I’m not familiar with Canadian tax law, but I assume so. Maybe there is a tax exemption limit for low-income earners. Probably your aunt knows more about this topic.
6. Will I lose my refugee status? If so, will it create a problem?
You are allowed to travel using your Blue Passport. You will lose your refugee status if you either travel to your home country or if you acquire another state’s citizenship. And as stated above: you might lose it if you stay abroad for too long without the agreement of Ausländerbehörde.
Check out this thread to learn more about the implications of losing your refugee status: https://www.wefugees.de/k/questions/2212
I hope this answer was helpful to you!