As requested, this is a more detailed video on how to merge two pieces of j channel together. Here I explain why you need to create a slip joint in j-channel and show 2 different methods to achieve said j-channel slip joint.
Here is an affiliate link to the snips I use: Andy 3” siding snips https://amzn.to/3QD05Zv
"Correction: 1:33 For Certainteed's method I stated to cut out 3/4" of the top rolled hem, this was incorrect - the over lap is supposed to be 3/4" and cutouts on the rolled hem and nail fin are both supposed to be 1"