Great question, so nodes that help secure the network receive ~15% yield on their T token (Threshold Network Token), creating incentive to continue running the nodes. We also have active overview of these nodes ensuring that they’re always up and operational.
We also have multiple audits and a bug bounty program. About & Threshold Network Bug Bounties | Immunefi | Immunefi
If you want to go into the specifics of wallet security there is a pretty comprehensive explanation that can be found here - Wallet Generation | Threshold Docs