How filebeat checks for new content in a file?

Does filebeat uses tail -f to check for new contents in a file and then flushes it over to the desired output? Or is there any other way it checks for new contents in a file?

Since filebeat is open source, you can always go look yourself

Here's the go code from the above linked file which checks if a file has been updated.

I've heavily abridged this code, anywhere you see ... is a code block that was not exactly relevant, I encourage anybody reading this too go look at the entire file, its some pretty well written go.

 // Scan starts a scanGlob for each provided path/glob
func (p *ProspectorLog) scan() {
	newlastscan := time.Now()
	// Now let's do one quick scan to pick up new files
	for _, path := range p.config.Paths {
		p.scanGlob(path)
	}
	p.lastscan = newlastscan
}

The above function gets called every n-length time block where n is specified in the configuration. ScanGlob gets called, and is shown below.

// Scans the specific path which can be a glob (/**/**/*.log)
// For all found files it is checked if a harvester should be started
func (p *ProspectorLog) scanGlob(glob string) {
	...
	// Evaluate the path as a wildcards/shell glob
	matches, err := filepath.Glob(glob)
	...
	// Check any matched files to see if we need to start a harvester
	for _, file := range matches {
		...

For all files which matched the glob, check the statistics on the file using the OS specific call, for linux this would be stat <file>

		// Stat the file, following any symlinks.
		fileinfo, err := os.Stat(file)
		...

Based on the stat call, it is decided if a harvester, the part of this go application which reads the files, needs to be started.

		// Conditions for starting a new harvester:
		// - file path hasn't been seen before
		// - the file's inode or device changed
		if !isKnown {
			p.checkNewFile(h)
		} else {
			h.Stat.Continue(&lastinfo)
			p.checkExistingFile(h, &newFile, &oldFile)
		}
		// Track the stat data for this file for later comparison to check for
		// rotation/etc
		p.prospectorList[h.Path] = *h.Stat
	}
}

TL;DR: Filebeat used the files statistics reported by the OS to see if the file has been updated since the last time it harvested the file.