120-days researcher short-term work permit exemption

[StetsonAB]

Hello there.

I got invited to a university for the graduate research assistant position for 3 months. Does anyone here have a similar experience and could enlighten me on how I should apply for the visa:

• which site is better to use: IRCC Portal (it demands a lesser amount of documents) or IRCC Secure (it demands way more documents such as family info, financial status, proof of assets and business, etc.)?
• do the invitation letters from the university and a supervising professor serve the purpose of "employment offer" and/or "employment contract"?
• etc.

Thank you in advance.