Do I need to add pectin to make jam?
I would probably just follow the recipes. If they're good, the amount of additional pectin they call for is what's necessary to get the jam to set well. If you use less than that, it might end up really runny.

Different kinds of fruit have different amounts of pectin, and it's not evenly distributed through the fruit. For example, apples and pears have a decent amount, while grapes and strawberries have very little. Oranges have some, but orange peel has a huge amount. There's a partial list in the Wikipedia article on pectin.

So, it's possible to make things that get the pectin they need from the fruit, especially if they're cooked enough to thicken and set more easily. Orange marmalade might be the most obvious example of this. But some recipes/fruits don't have enough to ever set, so they need additional pectin.

Since you're searching broadly online for recipes, you may also be seeing variations in how firmly set they come out. You might discover that some of the no-pectin or lower-pectin recipes just don't set as firmly, because that's what the author prefers. If the recipe comes with really good photos, you might be able to anticipate this.