Metal edging is a standard material for landscape planter areas and walkway edges. It is custom made for these locations. You can buy metal cut to the sizes that you need from a local steel fabricator. Typically they are cut to the lengths that you need and are 4 to 5 " deep. Typically 3/16" or 1/4" thick steel. There are some lighter gage metal edging available in pre-cut lengths at garden supply outlets.

I love the little squares around the trees. It gives the backyard a sort of edgy look that I love. I would love to have something like this in my own yard. https://luxurylawnslandscaping.com/services/landscaping