Here is a typical example from Vendr demostore where one wants to index node data from picked nodes. https://github.com/vendrhub/vendr-demo-store/blob/v3/dev/src/Vendr.DemoStore/Events/TransformExamineValues.cs#L44-L73

For example having a property with alias categories using a picker datatype (Content Picker, MNTP):

if (e.ValueSet.Values.ContainsKey("categories"))
{
    // Prepare a new collection for category names
    var categoryNames = new List<string>();

    // Parse the comma separated list of category UDIs
    var categoryIds = e.ValueSet.GetValue("categories").ToString().Split(',')
        .Select(x => UdiParser.TryParse<GuidUdi>(x, out var id) ? id : null)
        .Where(x => x != null)
        .ToList();

    // Fetch the category nodes and extract the category name, adding it to the names collection
    using (var ctx = _umbracoContextFactory.EnsureUmbracoContext())
    {
        foreach (var categoryId in categoryIds)
        {
            var category = ctx.UmbracoContext.Content.GetById(categoryId);
            if (category != null)
            {
                categoryNames.Add(category.Name);
            }
        }
    }

    // If we have some names, add these to the lucene index in a searchable way
    if (categoryNames.Count > 0)
    {
        values.Add("categoryNames", new[] { string.Join(" ", categoryNames) });
    }
}