Next.JS API Routes functionality behind the curtains

[zealous trench]

Hello there!
I have a question.

I am using an axios client within a Next.JS API Route:
const apiClient = axios.create({baseUrl: '...'})

Within the API route, I am setting a bearer token as default:
apiClient.defaults.headers.common['Authorization'] = Bearer ${accessToken}

After that, some async work happens, and then a request to the axios client is made.

Is each memory instance of Next.JS API routes completely separate when deployed to Vercel? Are they serverless lambda functions behind the curtains?
In other words, is it impossible that by settings apiClient.defaults.headers in one call of the API route, it would override the default headers in another?

[hard trail]

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[coarse badger]

it is possible I think

Vercel will by default bundle pages and bundle API routes into a single app

it can then split them intelligently but I couldn't get more details about the heuristics behind it

so I tend to consider that the API routes may run on different machines, process, or not

all the more that if you change your host you'd want your app to work the same

I don't see the point of axios in an API route to be frank

client-side routers are often used for their caching capabilities, so it may make sense if you extract Axios cache afterwards to feed a client context (if you use graphql this may make sense to use Dataloader this way for instance)