Jenkins has "console annotators" that affect how it wishes a log line to be displayed (html markup).
When browser requests log, Jenkins reads the log file off disk and gives chance to ConsoleAnnotator to annotate each log line with HTML markup. For example UrlConsoleAnnotator puts hyperlink markup (<a href=...>) on each URL string. Also see AnnotatedLargeText.
Not relevant to UI team but from log producer point of view, when Jenkins is writing log files, each log producer (filters) can provide their own ConsoleNote to generate annotated logs. For example WorkflowRunConsoleNote prefixes [Pipeline] to each workflow metadata related logs written by it.