PostgreSQL Memory consumption is too high

I need help in determining what is consuming so much memory in the cloud SQL (GCP managed) instance running with "PostgreSQL 16.6 on x86_64-pc-linux-gnu, compiled by Debian clang version 12.0.1, 64-bit".

Server spec: 16 vcpu and 128GB

Consistently, CACHE % is 59 - 60% and USAGE is around 39 - 40%. Therefore we have just around 0.5% free available memory. 

I ran queries to get shared_buffers and other values.

Here are the details:

shared_buffers: 42GB

work_mem: 4MB

maintenance_work_mem: 64MB

max_connections: 5900 (however, most of the times we have under 150 concurrent sessions)

There are no extensions installed.

Thank you in advance!

Regards

Siraj