I am saving them in my cold wallet and I'm wondering how to bridge my BTC to another chains such as Ethereum, Polygon to get more profit from Defi. Please help me and give me some good ways (reasonable cost and safe) to do that. Thank all!
Defi are not giving good profits anymore.
You will most likely ending having 3-5% per year or even less, and the risk is higher than you get in the tradicional markets, specially now with those high interest rates all over the world.
Additionally, some Defi are getting hacking almost every 2 months or so.
I would advice that you review your strategy.
If anyway you still want to continue, I recommend that you take a look in these links:
https://ethereum.org/en/bridges/