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When you know the "Type" you want a function to return (which in this case is "Id") you can search the standard library to see if any functions there return that Type using the input you want by pressing CTRL+F to bring up the word search bar and typing "returns TheTypeImLookingFor":

You can the cycle through what it finds and see if anything suits your fancy, I happened to find what I think you're looking for:

I'd also like to point out that a "Query" is just a map of maps of maps etc, and each layer is a subquery with a "queryType" key. If the queryType is correct then that map may also hold the Id of the entity that query refers too. I don't actually know much about how a query map is structured but you can always use "println(query)" in your code, open up a tab that uses that feautre, open up the notices and see how that specific query map is structured to see if it's something you can use. Good Luck

that would work, but I don't want to make a second, specialized mate connector FS just for this naming thing. I'm ok without it.

And of-coarse if you're trimming edges back you'll need to use "opEdgeChange" you'll have to figure out how to use it by studying the "Extend" tab in std library source: https://cad.onshape.com/documents/02e253006c3b752eb13303a7/w/7b120b69b752b11fdbbb09dc/e/4b13fb3967e27f942c8ae475

Actually I'll go one better: I've made an opTrimEdges function in case anyone wants to trim back the boundaries of a sheet/surface:

export function opTrimEdges(context is Context, id is Id, surfacesOrEdges is Query, trimDistance is ValueWithUnits, tangentPropegation is boolean)
{
    //get edges and track them
    var selectedEdges = qEntityFilter(surfacesOrEdges, EntityType.EDGE);
    if (tangentPropegation)
    {
        selectedEdges = qUnion([selectedEdges, qTangentConnectedEdges(selectedEdges)]);
    }
    var allEdges = qEdgeTopologyFilter(qUnion([selectedEdges, qOwnedByBody(surfacesOrEdges, EntityType.EDGE)]), EdgeTopology.ONE_SIDED);

    var trackedEdges = [];
    for (var edge in evaluateQuery(context, allEdges))
    {
        trackedEdges = append(trackedEdges, qUnion([edge, startTracking(context, edge)]));
    }
    
    //Make array to use in edge trimming
    var edgeChangeOptions = [];
    for (var i = 0; i < size(trackedEdges); i += 1)
    {
        var edge = trackedEdges[i];
        edgeChangeOptions = append(edgeChangeOptions, { "edge" : edge,
                    "face" : qAdjacent(edge, AdjacencyType.EDGE, EntityType.FACE),
                    "offset" : trimDistance});
    }
    if (edgeChangeOptions == [])
    {
        throw regenError(ErrorStringEnum.EXTEND_SHEET_BODY_NO_BODY, ["entities"]);
    }
    
    //trim edges
        try silent
    {
        opEdgeChange(context, id + "edgeTrim", { "edgeChangeOptions" : edgeChangeOptions });
    }
    catch
    {
        var edgesToTrim = [];
        for (var i = 0; i < size(edgeChangeOptions); i += 1)
        {
            edgesToTrim = append(edgesToTrim, edgeChangeOptions[i].edge);
        }
        setErrorEntities(context, id, { "entities" : qUnion(edgesToTrim) });
        throw regenError(ErrorStringEnum.TRIM_FAILED);
    }
}

Two more things to note that I've learnt is:

1. In std library the tab that holds the "moveBoundary" feature is called "extend" which is silly
2. You can refer to standard library tabs buy in this format "import(path : "onshape/std/##tab name##", version : "2543.0");" for example: import(path : "onshape/std/extend.fs", version : "2543.0"); and that way you can make sure you use the latest ver.