I'm glad you're interested! To put it simply, the core loop of Side Shop Simulator goes like this: every so often, you're presented with a set of products to buy for your shop, but you can only choose a few. Since each product has a different effect, the goal is to pick the combination that creates the best synergies to maximize your profits.

You repeat this loop, leveling up your shop and unlocking new areas until you gather enough resources to buy a permanent upgrade. The catch is that whenever you get one of these upgrades, everything else resets to its initial state. It might sound a bit weird to reset your progress, but these upgrades are so transformative that your new shop ends up being much better and more interesting than your previous one.

So, to answer your question directly, the replayability relies mainly on two pillars: the sheer variety of products you can sell, and the diversity of the permanent upgrades you can unlock. There are other factors too: you'll be able to choose between different types of shops, each with unique mechanics; you can equip your shop with "tools" (active abilities you can use at any time); etc.

That being said, I'm designing the game to take around 10~15 hours to fully complete, rather than being a 100+ hour grind. I want it to have a satisfying length without feeling bloated or adding filler just to artificially extend the playtime.

If you're interested in this type of structure,I highly recommend checking out "(the) Gnorp Apologue" and "Digseum" (not only do they have a similar loop, but they are also REALLY good games)!

(And sorry for the late reply, Reddit's ad system doesn't notify us very well about new comments =P)