How does trademark/brand registry works?

New seller here…just came across trademark/brand registry and got couple of questions:

1. let’s say I want to sell printing paper, I source my paper from China, I gonna do private labelling, I wrap it my own, registered UPC from GS1, put my own Brand name(not registered anywhere yet), can I still sell my printing papers? Am I violating any trademark/brand issue with Amazon? Lets say my own Brand name is not used anywhere, I crossed check everywhere including USPTO database

2. Printing paper all look the same, A4 A5 whatever size, they are white colour, any other brand will look the same, but just selling under my own packing and my own brand, am I violating any trademark/legal issue? for example if one big brand is selling printing paper, its trademarked/branded, does that mean only this big brand can sell printing paper but no one else?

3. let’s say my Printing Paper have a specific weight, or a watermark, looks like a Tiger for example. This other brand, is trademarked/branded have the exact same weight and watermark on his Printing Paper, am I violating trademark/legal issue because mine look identical to the other brand printing paper?

4. Shall I apply for trademark/brand straight away before listing my product? what if I want to test it out does it sell well before doing so? I saw this Product listed in around 2019, was doing alright then a year later it took off and became one of the most popular item in its category, the Product itself is just like Printing Paper, you won’t logo on the Paper but for the package, he trademarked his Brand/Logo in 2022 January, so does it mean he was testing his product for a year, it took off sold well then he started legalising it?

thank you very much