It won't hold a candle to some of these examples but for myself it's the POS software I currently work on.

What's that you say? How hard could it be? I can spin up a CRUD app where you can create items and add them to menus in an afternoon, and yes, you probably can do that but...

Are you going to make it so you can share options between items (Drink Size, pizza toppings, coffee flavor shots, etc)? Or are you going to manage those all independently on each item? That would be a pain when you go to add or remove 1 flavor from your 6 drinks that can have flavor shots added. Also think about reporting and/or stock tracking features. So instead you have "options" be another top-level entity that you can attach to items, problem solved!

But wait! Is "pepperoni" really $0.25 on all sizes? Or is it $0.25 on the small, $0.50 on the medium, etc? Ok so we need to have some way to adjust the price of the choices you select (pepperoni, sausage, etc) based on another option's selected choice (small/medium/large). Oh, and because you use different amounts of the toppings you are going to want to adjust the stock/recipe as well based on the selected choice. Let's also imagine the data structure for "gluten free is only available on the medium" and "stuffed crust is only available on the large", so we also need a way to disable other option's choices based on a previously selected choice. How about quantities on choices? 2x pepperoni? 2x espresso shots? What about 1/2 caramel shot? Are some options exclusive? Are some required? Do some have a max choice selection count? Do some have a minimum? Do some choices cost extra to add? Do you get 2 free selections then you pay after that?

Ok, ok, a little complex but we can handle this. What about discounts/coupons/etc? You'll need a way to apply a discount to an item or and order, ability to limit how many discounts can be used per item/order (different restaurants have different needs), ability to discount a flat amount or a %, limit which items a discount can be applied to ("$2 of any pizza"), limit which option-choices a discount can be applied to ("$2 off any medium pizza").

Ok, remember that menu we created? Do we really only have 1 menu? Or do we have a POS menu, an online menu, and maybe a doordash/Uber eats menu? Oh, and have fun converting your internal menu structure/concepts to the various food app's menu specifications. Do you want each location of your restaurant to have its own independent menu or do you want 1 top-level menu? Can you hide/remove items from your global menu on a per-store basis? Can you add items on a per-store basis (local specialty)? Do all your stores need to have a solid internet connection or function? (Spoiler, this is not a guarantee). Are you going to have local hardware/software so the store can function "offline"? Are you syncing menus down to these stores? Can your local hardware run the full stack without the cloud?

Are you accommodating for different types of restaurants? Drive thru? Delivery? In-store? Curbside? Table service? How about "little" things like "we want 2x Large coffee" or "we don't want quantities on items, they should all be separate"? Oh and those discounts? Might you want to also limit them based on the service type? This discount is for in-store only, this one is delivery only?

Do you have a loyalty program? Is it "points" or "stamps" based? Are there signup rewards? Are there visits rewards? How is your drip (or otherwise) marketing work? Are you making sure to only target people who aren't coming in every day/week/month already?

This and 1000 more things I never considered when thinking about POS software.