Most parts are replaceable, but I think at most only RAM and SSD are upgradable on the newer models. It's truly disgusting that that's become a feature and not the bare minimum.
On the T16 I have one soldered RAM slot and one replaceable. Of course the SSD can be upgraded too.
The P16 (not the P16s which is just a T16) has four upgradable RAM slots and two slots for SSD.
So that's not crazy amounts of upgradability but it's something.
I actually got a Legion Pro 5 instead because it's a lot cheaper but still has 2 upgradable RAM slots and two SSD slots. My only complaint is that the keyboard is less stable than on a thinkpad and it looks like a gaming laptop.
Someday I'll get a P16 though... or else one of those Eurocom laptops, if only they had 16:10 screens and nice keyboards.
On their site you can order a fully customized machine and they'll even install Linux for you.
I ordered a full AMD laptop with a lot more RAM by saving the Windows tax.
They are solid machines. I just got a Thinkpad P1 Gen 6 from work (1,8kg with a friggin 880 gram 230 Watt power supply that I will not have to carry around, fortunately) and it is great.