There are plenty of options: nix-sops, or nix-age, or whatever you would like - past the overall idea the implementation details are purely a matter of taste how you fancy things to be. Key idea is to have encrypted secrets in the store, decrypted at runtime using machine-specific credentials (host SSH keys are a typical option to repurpose, or you can set up something else). For local management you also encrypt to the “developer” keys (under the hood data is symmetrically encrypted with a random key and that key is encrypted to every key you have - machines and humans).

Alternatively, you can set up a secrets service (like a password manager/vault) and source from that. Difference is where the secrets live (encrypted store or networked service, with all the consequences of every approach), commonality is that they’re fetched at runtime, before your programs start.

I’m currently using deploy-rs, but if I’d redo my stuff (and the only reason I don’t is that I’m pretty much overwhelmed by life) I’d probably go with plain vanilla nixos-rebuild --target-host and skip any additional layers (that introduce extra complexity and fragility).

Just a crude and somewhat haphazard summary but hope it helps.