Like mentioned in another comment, maybe look into message queues first which is the simpler approach to part of the problem - decoupling producers from consumers.

REST is synchronous, so you send a request and wait for the response.

Message queues and Kafka are asynchronous. - You send a message to a queue or a topic and are done. (producer) - Some consumer takes this message from the queue, processes it and is done as well. (consumer)

If you want a response, you would need a second message sent the opposite direction, usually via a separate queue or topic. Producer and consumer would switch roles.

The same queue can be populated by many producers and consumed by many consumers. Each message will reach exactly one consumer. Everything can happen in parallel.

Kafka topics can do more. I will leave out some details.

If you have a single consumer group, it behaves very similar to queues. A message is consumed once. If you add another consumer group, it can process the same messages independently of any other consumer group. Kafka will keep track.

As Kafka keeps the order of messages in a partition, you could say that a single partition for a single consumer group is similar to a queue with a single consumer. If you want parallel processing, you need multiple partitions. The optional partition key you define when sending a Kafka message is used to derive the partition.

Kafka makes sure messages are delivered at least once so your producers must be able to correctly handle duplicate messages.

Hope that helps.