Upgrade 2TB SSD with 4TB SSD

You cannot upgrade the internal SSD in that MacBook Pro – or any current or recent Mac notebook, for that matter. The flash chips that hold your data are soldered to the motherboard – and part of the SSD controller is built into the main processor chip.

However, all of the 16" MacBook Pros with a hardware model year of 2023 have three multi-purpose USB-C ports which support

• USB 3.1 Gen 2 (up to 10 Gbps)
• USB4 (up to 40 Gbps)
• Thunderbolt 4 (up to 40 Gbps)

So you can add an external SSD, and there are a wide range of those to choose from, including some that are even smaller than portable mechanical hard drives. (Crucial offers some SSDs that are about the size of a credit card – with the thickness of an iPhone. Other World Computing is another vendor who sells portable SSDs . Both of them have portable SSDs avalable in capacities of up to 4 TB or 8 TB, depending on model.)

to add to Servant of Cats' reply, if you are just going to use it for storage, and especially if you don't need to constantly access the data on it, you could get an external HDD instead of an SSD to save some money.

I use a 2TB Crucial X9 (not the end of life Pro version) as an APFS-formattedTime Machine backup drive for my 16-in 2021 MacBook Pro M1 Pro. I chose it for its performance and diminutive size:

Crucial also makes this in a 4TB version that is presently $60 off at Amazon compared to the retail Crucial site.