Ah, got to love Erik's old answers :P
I'll write up a brief explanation - the use of `G` in that answer is actually really clever

```ṣ”|⁶ṁ;ɗ\pG  Main Link
ṣ”|         split on "|"
---ɗ\    cumulatively reduce (over last three as a dyad)
⁶ṁ       mold space like the current list (basically replaces the last item with spaces as well)
;      and append the current item
p   cartesian product with the comment item
G  format into grid```
essentially, it first splits on `"|"` into its components
for example, let's say we have `a|bc|def`
it becomes `["a", "bc", "def"]`
and then we cumulatively reduce that
so we start with the empty list on the left and `"a"` on the right
`⁶ṁ;` molds `" "` to the empty list (being empty of course) and then appends the `"a"`
so our first item is just `"a"`
on the next step, we have `"a"` on the left and `"bc"` on the right
`⁶ṁ;` molds `" "` to `"a"` so we get `" "` and then append `"bc"` to get `" bc"` as our second item
on the third step, we have `" bc"` on the left and `"def"` on the right so we mold `" "` and get `" "` and then append to get `" def"`
so now we have a list of the lines
then, we take the cartesian product with `"@"` (or whatever the comment character is)
so now we have `[["a", "@"], [" bc", "@"], [" def", "@"]]`
and finally we can just convert that into a grid which automatically spaces it out for us including (right) padding
to get
```a      @
bc    @
def @```

The `p` trick is a common Jelly golf if you need to append a character to the end of each list, and `G` is a clever way to get the required spacing

4:57 PM
wow