Felling Machine - Janome 9200D

[BrendaP]

Or this 

That a blind hemmer!

You don't need a special machine to sew felled hems though I think you can get special feet which do the turning for you.

https://www.youtube.com/watch?v=ODoLrCTxp5o
shows how to make a felled seam - using a regular overlock machine.   Be aware though that  at 2:36 she refers to the back of the fabric when she actually means back of the garment.

If you prefer not to press anything 
https://www.youtube.com/watch?v=KQ_JgFXhEaE
geths through it rather quicker!

In either of those methods you could use a twin needle for the second pass so that the left hand needle stitches in the ditch and the right hand one holds the folded fabric down.

If you prefer diagrams to videos
https://www.stitchpiecenpurl.com/flat-felled-seam-tutorial.htm

[Ohsewsimple]

They are felling machines evidently
https://www.sewingmachinery.com/used/blindstitch-chainstitch.html

[BrendaP]

Blind stitch machines have a curved needle which, when the tensions are correctly adjusted, just catches the fabric above a turned hem and the stitch is caught in place with a looper thread.

Industrial fellers for making felled seams (AKA lapped seams) are more like coverstitchers and can have one, two or three needles, but they have separate loopers for each needle and so make one, two or three lines of chain stitch but without the wide looper coverage on the back. 

See this one which has the special foot for folding the fabric.
https://www.sunnysewingmachine.com/index.php/view-all-machines/38-felling-machines/372-juki-ms-3580-off-the-arm-felling-machine-3-needle-chainstitch-like-union-special-35800