The other commenters are all right. But saying "it" instead of "they" here would be a very tiny mistake. You won't get shot for it.

Definitely 'they'. Jeans are plural, and the grammar has to follow. It would be wrong to use 'it'. Like pants, trousers, shorts and so on, this type of garment is always plural.

"They" would be the better choice, but "it" wouldn't be wrong either. They match my T-shirt and they're not expensive/ It matches my T-shirt and it's not expensive.

They for both of your examples.

Jeans. Has an s. Plural. So they is correct. Those jeans are perfect and they look great on you. It would not work in this case. If you are talking about the material of denim or jean then it qualifies as an identifier. Example, I like that style of denim jean material. Could you please pick it up for me? A bit tricky for sure. Good luck.