is it advisable to do pulumi IaC on ElasticCache Redis or should it be created manually in the Default VPC and not be part of IaC?

I'd avoid default VPC entirely if possible. the decision lies upon your use case and how many resources like this you'll need to produce. Ideally, I find it easier to read the code to learn about the relationship between resources rather than clicking through AWS Console. But there are cases where is it easier to create a resource manually and then inside pulumi just read the value (ARN or so,) from config or using

Get<resource>

function provided via the provider. Additionally, you can import an existing resource that was created manually and start managing it from pulumi. I hope this helps you to answer the question yourself.

Thanks Jan, yes I am planning to setup Redis and PostGresql and deployed thru IaC. This greatly helps!