Very simple GA4 API example in C#. See https://ithoughthecamewithyou.com/post/migrating-a-c-integration-from-ga3-to-ga4 for details.

GoogleAnalytics4Api.cs

// uses Google.Analytics.Data.V1Beta from NuGet
// credentials from Google cloud project authentication export
// GA4 Profile ID from GA4 Admin

var client = new BetaAnalyticsDataClientBuilder
{
    CredentialsPath = "PathToCredentials.json"
}.Build();

RunReportRequest request = new RunReportRequest
{
    Property = "properties/***YOURGA4PROFILE***" ,
    Metrics = { new Metric { Name = "totalUsers" }, },
    DateRanges = { new DateRange
        {
            StartDate = DateTime.UtcNow.AddDays(-31).ToString("yyyy-MM-dd"),
            EndDate = DateTime.UtcNow.AddDays(-1).ToString("yyyy-MM-dd") },
        },
};

RunReportResponse response = client.RunReport(request);
long users = Int64.Parse(response.Rows[0].MetricValues[0].Value, CultureInfo.InvariantCulture);