i want to develop a platfrom that monitor few things - which aws services i need to use for it?

Hello.

If you want to monitor on AWS, I think you will use CloudWatch.
There are several ways to register data to CloudWatch metrics, but if you want to monitor processes, I think you can do it by installing CloudWatch Agent.
https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch-Agent-procstat-process-metrics.html

If it is not running as a process, try running the API "put_metric_data" to create a custom metric.
https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/cloudwatch/client/put_metric_data.html

i want to make sure its always running .. and notified when there are errors

In the case of CloudFront, you can create alarms from standard metrics and be notified of what you want to monitor.
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/receiving-notifications.html

im serving cloudftont resources - .. i wanna be notified when there are serving errors

If MongoDB is installed on EC2 etc., please use the "put_metric_data" API to send the data you want to use for monitoring to CloudWatch metrics.

i wanna track profits trasholds from some remote mongo db - and notify when it hits lower the trashold

You can look on below link and make your own Observability

https://aws.amazon.com/cloudops/monitoring-and-observability/?whats-new-cards.sort-by=item.additionalFields.postDateTime&whats-new-cards.sort-order=desc&blog-posts-cards.sort-by=item.additionalFields.createdDate&blog-posts-cards.sort-order=desc

If not there are third party solutions like Datadog, new reliac who have in build features .. but it depends on scale you operating as there will be cost assosiated