Ask HN: Best practice for storing API keys securely in a B2B SaaS

My company builds B2B software that has integrations with other systems. These usually require an API key or similar.

When connecting to these systems, we need the API key as plain text.

What is the best practice for storing integrations API keys?

Storing it as plain text in our DB, might be risky in case our DB is compromised.

Thanks