Every session is anchored to a machine and a folder. That anchor is what makes the agent useful — it is working on real code, in a real checkout, with your real toolchain — and it is also the thing to get right before you start.
Choosing a machine
Paired Macs lists the machines your phone can reach directly. Remote Machines lists everything signed in to your account. Where one machine is doing most of your work, selecting it in the sidebar points every machine-scoped screen — Files, Console, Processes, Dock — at that machine at once.
Choosing a folder
Pick the folder the way you would pick a directory to run an agent in from the terminal: the root of the project, not your home directory. Narrower is better. An agent pointed at a single repository gives sharper answers than one pointed at everything you own.