You should tell us your laptop model number and ask for a recommendation for specifically what you want to add to the laptop. As of right now the question isn't really on topic but it can be if you provide that info.
I will edit my answer with a recommendation if you update the question.
There are a few standard expansion slots inside laptops. They are usually limited to a few specific kinds of components and almost all of the components in a laptop are built in to its motherboard in the first place.
This list isn't comprehensive but it covers most of the slots that have been common enough to have aftermarket components available to buy for them.
Old laptops had an external slot on the side for adding PCIe hardware.
Could be used for anything from extra USB ports to external GPUs.
Mostly for WiFi and Bluetooth modules in older laptops.
For older solid state drives.
For newer SSDs, WiFi/Bluetooth, maybe even more. (Very common in new laptops)
For replaceable graphics cards. (Very rare in high end older laptops like Alienware)
Thunderbolt 3 and USB4
This isn't necessarily inside the laptop, but these ports support PCIe connections and allow you to add basically any desktop PCIe card to a laptop by using an external adapter.