There is some overlap and there are a lot of differences. One point of overlap is that Nix makes it easy to install packages that are newer than your os. For example, you could run NixOS 20.09 or Ubuntu 16.04 and have the latest Handbrake or youtube-dl.
A large difference it that Flatpak also attempts to sandbox applications, which Nix doesn't (it only sandboxes builds). Though it might be easier to sandbox applications using Nix than traditional package managers, since all dependencies are made explicit.
A large difference it that Flatpak also attempts to sandbox applications, which Nix doesn't (it only sandboxes builds). Though it might be easier to sandbox applications using Nix than traditional package managers, since all dependencies are made explicit.