https://github.com/t184256/nix-on-droid/labels/android%20expertise%20needed lists some tickets that request clipboard features suggesting termux-clipboard as a workaround latter which progress is blocked for obvious reasons.

Ppl like me who can't wait or didn't know yet can also use i. e. nvim with an osc52 plugin to have at least that feature: copy from nix-on-droid to android's clipboard. For a possible setup follow these instructions.