I want to be a backend Java developer. What topics should I learn?

First off, forget "urgent". There's a lot to learn and it will take time. Hurry through the basics and you'll probably end up spending more time relearning later what you missed today.

Second, maybe you'll find a complete, free, high-quality video course online. I doubt it. The costs are just too large. Books are the best source of information for a sustained course of study. The problem with books is age. By the time a book gets through a couple of editions to correct errors and omission, the material covered can be a bit dated in tech terms. Still, the fact that developers are plunking down US$30-80 per book supports the effort of the author to provide a clear and reasonably complete presentation.

Text-based websites are second-best, I think, but can be pretty good. Sun Microsystems, which created Java long before being acquired by Oracle, created some really good online tutorials. Oracle has mostly maintained them; at least through Java SE 8 and Java EE 7.

For now, one really good place to start is the NetBeans tutorial trail on "Java EE and Web Development":
https://netbeans.org/kb/trails/java-ee.html

You'll need to use the NetBeans IDE with that option; and it's easiest to install either the "Java EE" or "All" version from the download site at:
https://netbeans.org/downloads/

Another option you can use instead of or in addition to the NetBeans tutorial is the Java SE Tutorial at Oracle.
https://docs.oracle.com/javaee/7/index.html

If you need to buff up your basic Java skills, the Java SE Tutorials have long been the best source of free, in-depth online learning:
https://docs.oracle.com/javase/tutorial/

Oracle seems to have abandoned support for online tutorials in the last year or so, though, so these will miss new features introduced in Java SE 9+ and Java EE 8. You don't need them right now, but eventually you'll need to buy books, take paid Oracle training courses, or learn from the (still free!) API Documentation.

Below are the Java 8 features

1. Functional Interface And Default Methods In Java 8 –

In this tutorial we will understand below topics- What is the use of Functional Interface in Java 8?
- When to use Functional Interface in Java 8?
- What is the need of static method in Interface ?
- What is the need of having a default method in an interface ?
PART 1 : FUNCTIONAL INTERFACE AND DEFAULT METHODS IN JAVA 8

2. Lambda Expression in Java 8
In this tutorial we will understand below topics
- What is Lambda Expression in Java 8?
- Lambda Expression with Parameters

PART 2 - LAMBDA EXPRESSION IN JAVA 8

3. Method Reference in Java 8

In this tutorial we will understand below topics
- What is Method Reference in Java 8?
- Types of Method References in Java 8?

PART 3 - METHOD REFERENCE IN JAVA 8

4. Optional in Java 8
In this tutorial we will understand below topics

- Why to use Optional in Java 8?
- What is Optional in Java 8?
- Different ways to create Optional?
- What is the Difference in between Optional.of() and Optional.ofNullable() ?
- Optional.of() vs Optional.ofNullable() ?
- Methods available in Optional

PART 4 – OPTIONAL IN JAVA 8

5. filter(), findAny() in Java 8
In this tutorial we will understand below topics

- How to use filter(), findAny() in Java 8
- filter – Under the hood

PART 5 - FILTER(), FINDANY() IN JAVA 8

6. map() vs flatMap() in Java 8
In this tutorial we will understand below topics

- map() in Java 8
- What is the use of flatMap() in Java 8?
- map() vs flatMap()
- flatMap() in Java 8

PART 6 - MAP() VS FLATMAP() IN JAVA 8

7. Stream.peek() in Java 8
In this tutorial we will understand below topics

- How Stream.peek() method is different from other methods?
- What is the use of peek() method in Java 8?

PART 7 - STREAM.PEEK() IN JAVA 8

8. Short circuiting operations of Streams in Java 8
In this tutorial we will understand below topics

- Short circuiting operations in Java 8
- Different Short circuiting operations in Java 8
- limit() in Java 8
- findFirst(), findAny() in Java 8
- allMatch() anyMatch() noneMatch() in Java 8

PART 8 - SHORT CIRCUITING OPERATIONS OF STREAMS IN JAVA 8

9. Advance Collectors in Java 8
In this tutorial we will understand below topics

- joining() in Java 8
- summaryStatistics() in Java 8
- partitioningBy() in Java 8
- groupingBy() in Java 8
- mappingBy() in Java 8

PART 9 - ADVANCE COLLECTORS IN JAVA 8

10. Intermediate and Terminal Operations of Stream in Java 8
In this tutorial we will understand below topics

- What is Intermediate and Terminal Operations of Stream in Java 8?
- Intermediate Operations in Java 8
- Terminal Operations in Java 8
- Difference between Intermediate and Terminal Operations in Java 8
- Intermediate vs Terminal Operations in Java 8

PART 10 - INTERMEDIATE AND TERMINAL OPERATIONS OF STREAM IN JAVA 8

Source Code
Download source code of Java 8 features from below git repository :
java-8-features
https://onlyfullstack.blogspot.com/p/java-8-features.html