how to get root folder of project where user start ?

Ivy puts all requests on a stack. There is an API to access this stack. But you can't directly access the initial process model version over Ivy.request(). I extended the example with the method findRootProcessModelVersionRequest(). This method finds the initial (root) ProcessModelVersionRequest.

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Optional;

import org.eclipse.core.resources.IFolder;

import ch.ivyteam.ivy.environment.Ivy;
import ch.ivyteam.ivy.request.IProcessModelVersionRequest;
import ch.ivyteam.ivy.request.IRequest;
import ch.ivyteam.ivy.security.internal.SecurityManager;

public class WebContentResourceLoader {

    public Path getResource(String relativeToWebContent) {
        return findResource(relativeToWebContent)
                .orElseThrow(() -> 
                new RuntimeException("Resource at webContent/" + relativeToWebContent + " cannot be found"));
    }

    public Optional<Path> findResource(String relativeToWebContent) {
        return getWebContentFolder()
                .map(f -> f.getFile(relativeToWebContent))
                .map(f -> f.getLocationURI())
                .map(u -> Paths.get(u))
                .filter(p -> Files.exists(p));
    }

    private static Optional<IFolder> getWebContentFolder() {
        try {
            return Optional.ofNullable(SecurityManager
                    .getSecurityManager()
                    .executeAsSystem(
                            () -> findRootProcessModelVersionRequest().getProcessModelVersion().getProject().getFolder("webContent")
            ));
        } catch (Exception failToGetWebContent) {
            throw new RuntimeException("Fail to reach webContent folder",
                    failToGetWebContent);
        }
    }

    private static IProcessModelVersionRequest findRootProcessModelVersionRequest() 
    {
        IProcessModelVersionRequest rootProcessModelVersionRequest = Ivy.request();
        IRequest request = rootProcessModelVersionRequest;
        do {
            request = request.getParentRequest();
            if (request instanceof IProcessModelVersionRequest) {
                rootProcessModelVersionRequest = (IProcessModelVersionRequest) request;
            }
        } while (request != null);
        return rootProcessModelVersionRequest;
    }
}

This will work if B is a Callable Sub Process. If you call B with a Trigger, this won't work. There will be a new task, which will ends up in a complete new request. In this case you have to pass the parameters through process data.

Note: You have to think about your design. Projects A depends on project B (A -> B). Now you create a implicit dependency backwards. A knows B and B knows A implicit. In my opinion that only makes sense, if B is something like a framework. Which seems to be the case - you call it "common components".