A data engineer designs, builds and runs the systems that collect data, store it, reshape it and hand it to the people and programs that need it. The output is not a chart or a model but a dependable supply of data: tables that are complete, correct, documented and refreshed on schedule. The title is newer than the work, which database administrators and "ETL developers" (extract, transform, load) did for decades; what changed in the 2010s was the volume and variety of data, more than one database and a nightly script could handle.
In small teams one person may do all four jobs, but the boundaries still tell you where a broken number should be fixed.