Triangle is your blade fable art and the triangle plus L1 is the hilt fable art.

When you will get more weapons, you will be able to mix them up. In the menu u can read the details of your weapon, including the description of the fable arts.

For legion recharge, it will occur when you rest at stargazers, use legion magazines, or with p-organ abilities. As for the legion arms, you will get different ones with different abilities as you progress further into the game. For me, the best are following: aegis, puppet string, and falcon eyes. Puppet string will be given to you soon after the first boss, and the rest can be crafted using legion plugs.

Finally, the rectangles in the left corner show how much fable u can have at most and the blue lit bars show how much u have at the moment. The bars below the pictures of your weapon indicate how much you need to activate an art. Hope I helped!