Advice for shelving unit build out?

Hello!

I've created an imgur album to show what I am trying to build. Unfortunately, the reference photos are pretty awful. The photos are from a house-listing so they've been cropped extra close for privacy reasons.

I tried to create diagrams based on what I can remember from being at the open house, but it was a year ago and the details are fuzzy. Apologies about the number of ways I've misspelt shelves or other words in the notes.

I've got some prior woodworking and DIY handyman experience, but have never worked with metal before, so my questions are specifically around metal materials lingo:

• The legs of the shelving unit look like they might be some kind of L-shaped stainless steel. But I don't know the terminology here, and spending time on Lowes/Home depot website hasn't helped. Any thoughts what that metal might be called? Nevermind just found them! 'Perforated Angle'

• Cross-beam things. Ikea uses them, other furniture stores use them for stabilization. But I've never seen them at a hardware store before, which likely means I don't actually know what they're called, or where to find them. Again, what might these things be called?

• Support for the shelves ... I'm going to hazard a guess that the standard plastic pegs that come with cheap-o furniture and adjustable shelves isn't going to cut it here. But the holes in the leg/poles look pretty large for any kind of standard furniture peg... ideas?

• We don't want this thing to weigh a ton, but it does look like there are some solid metal pieces used on the bottom half of the siding, and possibly in the middle section as a divider as well. Would this also be stainless steel? Or something different? I think if steel as being relatively heavy. Herp derp. I'm dumb, aluminum sheets!

• Related to that: for a back stop-bar (to prevent things from going of the back end) is there something better than metal that can be used? I don't believe wood will look right as a cross bar, or really be a wise alternative (also weight). Is there a prefab material that is light that might work as an alternate?

Edit to add other questions I forgot / when answers found.