By the way, this page is 404: https://just-comments.com/about.html (and some others from ToS)
Otherwise, congrats on the launch and good luck!
The tech is completely hosted on AWS, and it is entirely "serverless". So the core is using the API Gateway + Lambda + DynamoDB. Additionally, I use Kinesis Firehose + S3 to process internally generated data such as billing events. Athena helps me to query the data if needed.
Basically, the scalability & high availability is entirely provided by AWS and the pricing model matches almost 1-to-1 the pricing by AWS.
If you have any specific questions about the tech, I can answer them.
Just being curious about your plans
Generally, the project is none-free and low-margin and that is what makes it interesting. There are plenty of dead and alive open source projects offering comments which you can use.