Learn French: Marchez sans toucher quoi que ce soit ! - Walk without touching anything at all!

You could definitely say Marchez sans rien toucher (Walk without touching anything). However, the phrase quoi que ce soit adds strong emphasis, translating closer to "anything at all" or "whatsoever". It stresses that there are absolutely no exceptions to the rule.

How do I know whether to use quoi que ce soit or qui que ce soit?

It depends on whether you are talking about a thing or a person. Use quoi que ce soit for things ("anything at all") and qui que ce soit for people ("anyone at all"). Here, the instruction is about not touching objects, so we use quoi.

Why is it sans toucher (infinitive) instead of a present participle like the English "without touching"?

In French, when a verb immediately follows a preposition like sans (without), pour (for/in order to), or de (of/to), it is always left in the infinitive. English uses the "-ing" form after prepositions, but French strictly uses the infinitive.

I recognize soit. Why does this phrase use the subjunctive?

The expression quoi que ce soit literally translates to "whatever it may be". The phrase quoi que (whatever) introduces an indefinite, hypothetical idea rather than a concrete fact, which is a classic trigger for the subjunctive mood in French.