shared-buffers set to 24GB but the RAM only use 4-5 GB average

I have configured postgresql.conf with parameters as below:

log_destination = 'stderr'
logging_collector = on
log_directory = 'pg_log'
listen_addresses = '*'
log_destination = 'stderr'
logging_collector = on
log_directory = 'pg_log'
log_rotation_age = 1d
log_rotation_size = 1024MB
listen_addresses = '*'
checkpoint_segments = 64
wal_keep_segments = 128
max_connections = 9999
max_prepared_transactions = 9999
checkpoint_completion_target = 0.9
default_statistics_target = 10
maintenance_work_mem = 1GB
effective_cache_size = 64GB
shared_buffers = 24GB
work_mem = 5MB
wal_buffers = 8MB
port = 40003
pooler_port = 40053
gtm_host = 'node03'
gtm_port = 10053

As you can see, I have set the shared_buffers to 24GB, but my server
still only use 4-5 GB average.
I have 128GB RAM in a single server.
My database has 2 tables:
- room (3GB size if pg_dump'ed)
- message (17GB if pg_dump'ed)

The backend application is a messaging server, in average there will
be 40-180 connections to the postgres Server.
The traffic is quite almost-heavy.

How to make postgres-xl effectively utilizes the resource of RAM for
9999 max_connections?


Thanks,
FattahRozzaq