How do I know which strap width fits my watch?

Storage / memory: 128GB/8GB
Condition: 8/10
Included: USB cable

What matters is not the case size but the gap between the lugs, measured in millimetres — most often 18, 20 or 22 mm. Measure between the lugs with a ruler, or check the watch's specification. Apple Watch and some Samsung models use their own fitting, so they need a strap made for that model.

Does a smartwatch need its own SIM card?

Most do not. Standard models connect to the phone over Bluetooth and use its connection. A separate eSIM is only needed on LTE versions, if you want to take calls and messages with the phone left at home.

What does a watch's water resistance actually mean?

5 ATM means the watch will survive rain, washing your hands and swimming in a pool, but not diving or a hot shower — steam damages the seals. IP68 means protection against dust and brief immersion, but it is not meant for swimming. Leather straps do not survive water even when the watch does.