With @types/node installed and strict on, what is the type of port?

The type is string | undefined. Node's types declare every process.env value this way, for two reasons:
• Any variable may be unset, so it can be undefined.
• Environment values are always strings.

It is never number. A common pattern is to parse process.env once at startup with a schema, for example z.coerce.number(). Then you export a typed config object for the rest of the app.