Powershell pipelines are an engine construct rather than OS pipes or file descriptors. (If you include OS binaries in a PS pipeline, it will map the internal pipeline to OS pipes for that element of the pipeline, of course.)
Every Powershell command has a begin, process, and end block. (If you don't write these explicitly, your code goes in an implicit end block.)
When a pipeline is evaluated:
1. From left to right, the begin block of each command is run, sequentially. No process or end blocks are run until every begin block has run.
2. Each command's process block is run, once per object piped in. A process block can output zero, one or many objects; I'd have to check on a computer, but IIRC this is "breadth-first" - each object that a process block outputs is passed to the next process block before returning control to the current process block.
3. After all process blocks are exhausted, from left to right, each command's end block is run. Commands that did not declare a process block receive all piped objects as a single collection. Any output from the end block triggers the process block to the right.
4. When all end blocks have completed, the pipeline is stopped
5. Errors in Powershell can be terminating or non-terminating. When a terminating error is thrown, the pipeline is stopped
6. There is a special StopPipeline error which stops the pipeline but is handled by the engine so the user never sees it. That's how `select -First 5` works (for PS `select`, not gnu select).
Pipelines only operate on streams 0 and 1, as with OS pipes. The other streams (ps has 7) are handled immediately, modulo some buffering behaviour intoxicated for performance reasons. Broadly speaking, the alternate streams are suppressed or enabled by defaults and by switches on each command individually and are rendered by the engine and given to the console to display. But they can also be redirected or captured in variables.
You can do asynchrony in Powershell; threading is offered by a construct called "runspaces". These are not inherently connected to the pipeline, but pipelined commands can implement them, e.g. `foreach -Parallel {do-stuff}`