HP and Lenovo theoretically do, but you've got probably 50%+ odds of the part you want either being perpetually unavailable/OOS or priced insanely if you try to buy direct.
I don't know about Dell but I put several original and not original parts inside my HP laptop and they worked well. HP's next business day service came to my home twice to replace the screen and the keyboard. I replaced the keyboard myself after the 3 years service period expired. I never bought from HP, just googled the part number and picked one of the results. Anyway I think HP sells through partners here.
I increased RAM twice, one rightaafter unboxing the laptop because buying an 8 GB laptop plus 16 GB from a third party was some hundreds Euros cheaper than buying a 16 GB laptop from HP. I bought two SSDs in different years and replaced the spinning disk and the DVD. I can't see myself having to depend from a single company for such things, and some of them being outright impossible.
Dell doesn't sell most parts to the public.
HP and Lenovo theoretically do, but you've got probably 50%+ odds of the part you want either being perpetually unavailable/OOS or priced insanely if you try to buy direct.