They likely have some sort of database they can access like the other credit card companies to estimate your income but they don’t normally ask for POI unless you’re asking past a certain threshold, I believe 25K, or your stated income is considered very high.
Otherwise its on the honor system like most credit card companies, a rarity for credit unions.
I can’t see them asking for your income. I have never had them request income verification and I get $18K a year which is what I reported to them - I have $15.2K in credit with them since I joined in September.
The times I have seen are large CLI requests like you mentioned or very high reported income.
I wouldnt recommend trying to get credit based on temporarily inflated income either.
I recently did an auto load refi with NFCU, and had to upload pay stubs to a secure document portal on their site.
I've pondered this myself just in case it ever does happen, I too have fluctuating income from to month. So I would prefer using W2's as it correctly reprisents what I state that I earn annually. However, if a finacial institution ever requested a pay stub during one of my smaller paychecks. It may look fishy to them as the math wouldn't add up. Though I haven't been as for POI in a very long time, since I was younger and much lower credit scores.