How is the maven version specified in Spring Boot Starters?

I've read the reference but I don't understand how versionId is set in the dependency. For example the MongoDB starter specifies

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mongodb</groupId>
            <artifactId>mongodb-driver</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-mongodb</artifactId>
            <exclusions>
                <exclusion>
                    <groupId>org.mongodb</groupId>
                    <artifactId>mongo-java-driver</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>org.slf4j</groupId>
                    <artifactId>jcl-over-slf4j</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>

But the version tag is missing. How is the version of mongodb-driver determined when using these starts?

This is related not only to starters, if you have many modules in your project they have different dependencies. As regular practice, there is a parent pom.xml with general settings.

If you see a dependency in child maven module like below:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
</dependency>

There is a configured dependency in parent pom like:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.5</version>
</dependency>

Usually, it's more flexible to configure versions with properties:

<properties>
    <commons-lang3.version>3.5</commons-lang3.version>
</properties>

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>${commons-lang3.version}</version>
</dependency>

MongoDb version is defined in spring-boot-dependencies using Dependency Management mechanism.

<properties>
    <mongodb.version>3.8.0-beta2</mongodb.version>
<properties>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.mongodb</groupId>
            <artifactId>mongodb-driver</artifactId>
            <version>${mongodb.version}</version>
        </dependency>
    <dependencies>
<dependencyManagement>

Dependency management is a mechanism for centralizing dependency information. When you have a set of projects that inherits a common parent it's possible to put all information about the dependency in the common POM and have simpler references to the artifacts in the child POMs.