wal writer process vs writer process vs checkpoint process

Hello

My customer asked me about the relationship about PostgreSQL's following
process:
wal writer process writer process
checkpoint process

Currently My understanding is:
If I execute some DML, then,Firstly , the related operation or data will be
written to wal buffer.
Secondly, the related data will be written to data buffer(shared_buffer).
And because the above data structure, Then
wal writer process will periodically read from wal buffer, then write them
into wal log.
writer process will read from data buffer and then write them into data
file(eg: table).
And checkpointer process will read from data buffer then write them into
wal log according to checkpoint_timeout and checkpoint_segments settings.
Is my understanding right?
Thanks!
Jian Gao