Hardware for scanning diapostives or negatives using flatbed scanner?

Flatbed scanners with film scanning capability have not completely disappeared from the market. You are correct, scanning film requires the film to be backlit. So scanners with the capability have an illumination source on both sides of the scanner platen. It is (sometimes?) called a "Transparency Unit". "Back in the day", that's how it worked, and that's still how it works.

You won't find a multifunction printer/scanner with film scanning capability, as far as I know. You'll need to buy a scanner with film scanning capability, and it will leave you with more flexibility in choosing a separate printer (if anyone still needs a printer in 2021).

You can search on a website like Amazon or B&H, or visit manufacturer websites. Take a look for example at Epson's "Perfection" scanners, particularly those with "V" in the model name.

Also I guess the best way to understand how it works is a search on YouTube.

Photographing negatives with a digital camera is an alternative to a flatbed scanner. For on screen use, a even a modern smart device camera may be good enough.

Because negatives and slides modulate light by transmission rather than reflection, capturing their information effectively requires a light source behind the negative. Bespoke products are available for purchase from companies such as Lomography and do it yourself solutions are feasible.

The web is a great source of ideas and information about “scanning“ negatives with a camera. It is probably the most common method in use these days because digital cameras are so common.

In case you have a sufficiently large amount of pictures, I recommend buying specialized hardware with an automated feeder. 16 bits per channel is an absolute necessity! Depending on your use case, you may want to rent one rather than buy one. I used a https://reflecta.de/en/computer-required/23-reflecta-digitdia-7000-magazinscanner.html (I am not affiliated with the brand) and it was well worth the money (rented one week for 150 EUR, processed more than 2500 pictures).

My (fairly old) Epson V200 scanner, as set up to scan negative/slides:

Specs says it can scan at 4800DPI, so you can get a 30Mpix image out of a 35mm slide/negative (personally never went that far). Epson still has photo-capable scanners in its line-up.