They're not really "complex securities"; they're just convertible bonds. The lender gets to choose if they want to be paid back (1) in a fixed note value (like a bond), or (2) with shares of the company (like equity)—if it IPO's. (I.e., the value is the max() of the two. ReLU-shaped).

https://en.wikipedia.org/wiki/Convertible_bond

I don't understand the accounting aspects, and they seemed very weird to me, when I tried to read about was happening with Anthropic. Basically: the rules says the convertible debt has to be valued at the predicted IPO price—because that's the value of the thing (Anthropic stock) Anthropic will be obligated to pay. At the point they re-calculated this debt, the valuation went up drastically, so the rules say that counts as a very large quarterly "loss". But: on the other side of the scale, the increase in predicted IPO price does not count as a quarterly "profit"—even though they should cancel out. Even though the increase in share price exactly cancels the size of the debt they're paying back—the debt denominated as a fixed number of shares.