Old SQL to the New. table by table joins - oracle

Question

With our oracle Database/queries that are currently running i have come across some SQL where they have done a table by table join. Now I want to be able to understand this so could someone explain? I am a newbie to this.
SELECT *
FROM ra_customer_trx_all
WHERE customer_trx_id IN
(SELECT customer_trx_id
FROM AR_PAYMENT_SCHEDULES_ALL
WHERE payment_schedule_ID IN
(SELECT payment_schedule_ID
FROM AR_RECEIVABLE_APPLICATIONS_ALL
WHERE applied_customer_trx_id =
SELECT customer_trx_id FROM ra_customer_trx_all WHERE trx_number = '34054'));

Answer 1

1st:
select all TRX records from table ra_customer_trx_all where number = 34054
we are looking for customer_trx_id
select * from ra_customer_trx_all t4 where t4.trx_number = '34054'
2nd: select all records from payment_schedule table that have the IDs from step1
select * from AR_RECEIVABLE_APPLICATIONS_ALL t3 where t3.payment_schedule_ID = (prev select)
3rd: select all records from customer_trx_all table that have the IDs from step2
select * from AR_PAYMENT_SCHEDULES_ALL t2 where t3.customer_trx_id = (prev select)
4th
select * from ra_customer_trx_all t1 where t2.customer_trx_id = (prev select)
5:
summary:
if trx is transation
the logic is:
select all customer transaction records that have been scheduled to be paid via the RECEIVABLE_APPLICATIONS and transaction number is 34054
SELECT t1.*
FROM ra_customer_trx_all t1
inner join AR_PAYMENT_SCHEDULES_ALL t2 on t2.customer_trx_id = t1.customer_trx_id
inner join AR_RECEIVABLE_APPLICATIONS_ALL t3 on t3.payment_schedule_ID = t2.payment_schedule_ID
inner join ra_customer_trx_all t4 on t4.customer_trx_id = t3.applied_customer_trx_id
where t4.trx_number = '34054'

Answer 2

You can replace
select *
from tableA
where columnA in (select columnB
from tableB
where columnB1 in (select ...))
with
select *
from tableA, tableB
where tableA.columnA = tableB.columnB
and tableB.columnB1 in (select ...)
Apply this pattern sequentially to each subquery.
Short explanation: you open outer brackets after IN keyword, move table from inner FROM clause to outer, and add condition to WHERE clause: column before IN have to be equal to column in SELECT clause in subquery.