For example, in this speedrun: They allow the runner to go through character creation before they start the timer.

What parts of game set-up am I allowed to do in this game before I'm supposed to start the clock?

As soon as you gain control of your sim when the map zooms in on your lot the run starts. (After character creation and picking a lot)

[ * TS3 & TS3 Category Extensions: General Rules *

• Timer starts whenever the camera zooms into the lot ]

^ From the official category extension rules pastebin

What parts of game set-up am I allowed to do in this game before I'm supposed to start the clock?

This can be answered easily from the General Rules that can be found on: https://pastebin.com/raw/2nnttLKd or on https://www.speedrun.com/ts3/thread/egqr4

You are allowed to do the following:

• You can either, start from a house that comes with the town, or build a house yourself
• Your Sim must be freshly made, you can not say use one from the Exchange or Store etc.
• Seeding of the Neighborhood for Family Friends is allowed

But note that individual categories will have specific rules. Please read those before starting or submitting.

What is seeding?

Seeding - Meaning placing Families closer around where your character is going to live in order to make Friends quicker

I see, thank you