Definitely not something you can train and, once you do go down the path of having it investigated, number one advice from the professionals is that you DON’T lift /wake at night.
My seven year old is still soaking wet at night (the pull-up hits the bottom of the bin with a thump every morning). His almost-five year old brother has been dry at night since he was two. The hormone has kicked in for the younger, but not the older.
DS1 has been under a paediatrician (since he was six, so they do investigate from younger than seven) and tried on both of the meds they use to bring about night dryness with no change whatsoever. Our next step is an alarm which I honestly don’t think I’m going to bother with. That child robbed us of far too much sleep when he was a baby/toddler to start that all over again now. I also think that his night wetting is related to the fact that he sleeps so heavily (oh, the irony, given the multiple night-waker he once was) and the alarm would wake everyone except him.
The only things you can reasonably try is to really increase day-time fluid consumption, in case it’s an issue around bladder size- drinking lots (i.e. at least 1-1.5L per day) stretches the bladder, theoretically meaning it can hold more at night before being released. No fluids at all after 6pm, not even bedtime milk if you’re in the habit of this - if they’ve taken in the amount as above, this shouldn’t be an issue. Also, double-wee before bed - have one wee, then bath, pyjamas, stories, and try and squeeze out one more wee.
None of the above worked for my DS or his classmate with the same issue, so now it’s just a waiting game. His teacher’s son just suddenly stopped being wet at night at the age of nine, so I’m hoping it will all just sort itself out one day, now that we’ve tried most things.