analoge output into Atom analoge-in. (Maybe not optimal, from sound quality perspective.)
digital output into Atom digital-in. (Recent Macs don’t have digital-out anymore; so you would need some USB-C-device or Thunderbolt-Dock or something to provide digital out.)
you can stream audio from Mac to the Atom via AirPlay;
either via Music App (which only relays the music and can target multiple AirPlay targets)
the macOS audio-redirect via AirPlay (sends the whole system audio from any app via AirPlay, can target only a single device)
This introduces some delay (2 seconds?), so it’s fine for listening to music, but not for any interactive usage (like gaming, audio editing) or watching movies. For these cases, you should get a direct (cabled) audio connection.
I think there’s 3rd party apps for more fancy setups. (I did not have the need to look into this yet.)
Do you want to connect a DAC to the Atom? (It has an internal DAC and no useful method to use one, IMHO.)
Or do you want to connect a DAC-output into the Atom? (You could do this via analoge-in, but then it goes through the Atom internal ADC/DAC chain again, which kind of makes this a questionable use case.)
If you have digital sources, you should connect them to the digital-in of the Atom, or stream music to it. (That’s how it’s intended to be used.)
The USB ports on the Naim Unitis are for adding stick or HDD storage, not for connecting to a computer like a USB DAC. You are best off streaming from your computer over your network. If you do wish to directly connect to a Unit from a computer without using a network then you’ll need to use one of the s/pdif inputs (I would recommend using a good quality asynchronous USB to s/pdif convertor here).
Naim streamers are primarily network players, and all you really need is an Ethernet connection (or WiFi.)
Qobuz is supported natively by Naim streamers, so you don’t need a computer in the chain.
If you have music stored on your computer you can run a UPnP server on it, and the Atom will find the files and play them. Audirvana can do this, or try Asset or Minimserver, or if you use iTunes/Apple Music, there’s AirPlay.
Another easy way to play locally stored music is to copy it to a USB drive and connect this to the Atom USB port.
You would use a server (for example, Asset) loaded on your computer (or elsewhere) to serve any music stored on your MacBook, or elsewhere on your network, to the Uniti. The Uniti is a network player with an amplifier all-in-one box.
However, if you wish to connect directly, thus using your MacBook as a source and the Uniti just as a DAC and amplifier then you’ll need a suitable asynchronous USB-S/pdif convertor. There are many available, and many here had good results using M2Tech, Audiophilleo, Gustard etc…