That's always a good idea to go and look and get a feel for things. So that's the G15 ruled out then
One Dell Precision 16 model did have onboard ram that wasn't upgradeable, but that was the one that didn't have Nvidia Graphics. It the site shows upgradeable ram then I'm sure it's correct. But it might be things have changed recently. If it is the case that the ram isn't upgradeable on any of them, you might as well get the less expensive one with the Arc graphics. Except it does limit you to 16gb of ram, which, in 5 years time you might regret. So in that case, Glaskutter's advice to get one with 32gb of ram now, was good. Except that will cost a lot more.
I don't know much about Lenovo laptops as I've never had one - some are recommended in reviews. I tend to stick with HP because they're familiar to me and I like the keyboards. I like Asus keyboards as well but their screens mostly have PWM and I avoid that. Most HP's don't have PWM. It's usually the Oled screens that do (I think) and I'm not bothered about an Oled screen. Any screen will do as long as it's not terrible!
Maybe you could call Dell or email them about the specs for the 16 with Nvidia and just check that it does still have upgradeable ram - it does seem to suggest that on the site and ram is available to buy for that machine. So either the sales assistants are incorrect/not up to speed - or they are ahead of the game and know the specs have changed from what is on the site!