Outsourcing usually gets discussed as one decision, but it is really two different models wearing the same label: staff augmentation, where individuals slot into your existing team, and a dedicated pod, where a self-contained team runs a workstream with its own internal structure.
Staff augmentation fits when you have the structure already
If you already have a project manager, an established process, and a specific skills gap — you need two more senior engineers, not a whole new team — augmentation is the lighter-weight option. The people you bring in report into your existing structure and ramp up faster because they are following processes you already trust.
A dedicated team fits when the work is self-contained
When the work is a distinct product or workstream — a new mobile app, a platform rebuild, a project running in parallel to your core team — a dedicated pod with its own PM, engineers, and QA can move independently without constantly pulling attention from your in-house team.
The question that actually decides it
Ask who would own the day-to-day decisions on this work if it were fully in-house. If the answer is "someone on our team, just with more hands," augment. If the answer is "a dedicated lead who reports back to us periodically," go with a dedicated team. Getting this wrong is the most common reason outsourcing engagements feel like a bad fit.