How to properly build walls under windows? Galvanized angle iron?

aha... ok now I have no idea whether he did that :-/

Well, I hope so.

Uh, galvanized iron is surely just stainless steel but galvanized?

Certainly not. Firstly, there is no reason to galvanize stainless steel (=rust-free steel) here because it does not rust. Secondly, zinc on stainless steel leads to contact corrosion (cf. electrochemical series).