Question: I Am trying to learn the RDBMS. I have question for you guys. Why does a DBMS interleave the actions of the different transactions instead of executing transactions one after another?

Answer: 
A DBMS is typically shared among many users. Transactions from these users
can be interleaved to improve the execution time of users’ queries. By interleaving
queries, users do not have to wait for other user’s transactions to complete
fully before their own transaction begins. Without interleaving, if user A begins
a transaction that will take 10 seconds to complete, and user B wants to begin
a transaction, user B would have to wait an additional 10 seconds for user
A’s transaction to complete before the database would begin processing user B’s
request.
The transaction can be defined as the group of tasks or the program unit which after execution changes the contents of a database.
To decrease waiting time of users DBMS uses concurrent transactions.
[DBMS Transactions](https://whatisdbms.com/explain-dbms-transactions/)