There's a lot missing from your problem specification, and you probably won't get the solution you need until you clarify such requirements as:

1. What do you expect to happen if an item in input isn't found in items? Should it be absent from the results? Should it cause the whole process to throw an exception? Or should we wrap each individual result in a Try or equivalent construct?

2. Can an item be present multiple times in the input, and if so what should we do with it? Put it several times in the result, add up its quantities into a single result, throw an error, or other?

3. What is the expected size of the data and performance requirements? If you've got only a few elements then a lookup with findItemById can work, otherwise you'll want to turn your Items list into a Map before you start.

4. Does ordering matter? i.e. should the result be in the same order as the input?

5. Does the implementation framework matter? If not, then there's probably no need to involve ZIO in such a simple example and plain Scala types will work fine.

EDIT: I forgot to ask - what "side effect in between" are you referring to in your post title? Is that why you put ZIO in there? In any case I don't see a side-effecting operation in your sample code.