How to investigate active MySQL transactions that are not visible in processlist?

We have a Java application that works with MariaDB 10.4.28 hosted in AWS RDS.

We have a situation that application went down with reason "CannotCreateTransactionException". The pool has 40 active connections and 0 idle connections.

We have several tools that fire at such moments to help debug such situations:

ThreadStackDump
- The dump shows that there are no actual threads running any queries at all, all threads are waiting for a connection from a pool.
"show processlist"
- It shows no queries running except just a couple normal ones that are not hanged and finish within milliseconds
"SHOW ENGINE INNODB STATUS"
- It in fact shows a lot of records like the following

---TRANSACTION 2974372519, ACTIVE 31 sec
  2 lock struct(s), heap size 1128, 0 row lock(s), undo log entries 4

Questions:

What can be a command or an approach to understand that this 2974372519 transaction is, what it is attempting to do?
What can be an approach to understand what my 40 occupied DB pool connections are doing?

We use HikariCP:4.0.3 and mysql-connector-java:8.0.22.

Solution

Transactions are stored in INFORMATION_SCHEMA.INNODB_TRX. To find the statement, just execute

SELECT trx_query FROM INFORMATION_SCHEMA.INNODB_TRX WHERE trx_id=2974372519;