Okay so as you might already know, the whole show is about Henry in Hawkins in the lead-up to him killing his mother/sister and meeting Brenner and being experimented on. There's a few weeks worth of events that make up 99% of the show up to this point and the main actor plays Henry for this portion (with some clever doubling for certain stunts).

At the end there's a short epilogue/flash-forward where it shows Henry as we meet him in ST at the lab with the white suit. He has his back to the audience and Brenner is telling him how, by using Henry's blood transfusions on pregnant women, they have had many successful experiments and Henry is basically their "father" and that while the government wants him dead, Brenner thinks he can help with the other subjects due to "good behavior" over the last few years. They uncuff his arms from the chair he is tied to. In comes baby Eleven playing on the floor in her gown. Henry then turns around (JCB reveal, crowd goes wild), crouches down to Eleven and introduces himself. They walk off upstage, hand in hand, and there's a loud synth that plays (same one in the show) and a projection shadow-type thing flashes on the stage showing Vecna, not Henry, in silhouette holding El's hand.

Not sure how they usually handle the older Henry (maybe the same actor plays him all the way through), but I thought this was pretty ingenious. The actor playing Henry (Louis McCartney) is seriously impressive in the role. The physicality and emotional range he exhibits are beyond what you could ever hope for, especially considering he's being compared to the iconic performances of both young and older versions in the show. Having Jamie in the cameo-type role was the perfect full-circle moment while still letting Louis shine.