The industry standard (IICRC S500) sorts water into three categories by how contaminated it is. Category 1 is clean water from a burst supply pipe or an overflowing tub. Category 2 has picked up some contamination, from a dishwasher, a washing machine or a sump pump. Category 3 is grossly contaminated water: sewage, a toilet that overflowed with waste in it, or floodwater from outside. The category is judged on site, and clean water left standing does not stay clean. It decides what can be dried in place and what has to come out.
Standing water is pumped out first. Then the technicians map where the water went with moisture meters and often a thermal camera, write down the readings, and measure again every day until the materials read dry. Materials that cannot be dried, such as soaked carpet pad, swollen particleboard and drywall touched by Category 3 water, are removed. Fans and dehumidifiers run until the daily readings match a normal dry reading for each material. Drying is finished when the numbers say so, not when the surface feels dry.
In a home built before about 1980, drywall compound, plaster, acoustic ceilings and floor tiles can contain asbestos, and older homes may have lead paint. The South Coast air district requires an asbestos survey before most renovation and demolition work, and asbestos removal is a separately certified trade. A crew that starts cutting out walls in an older home without asking about this is a warning sign.
Mold is the reason speed matters. Public health guidance says to dry wet materials within a day or two to prevent mold growth, so the goal is to have drying under way inside that window. That is also why a slow leak found late is often a mold job as well as a water job.
For insurance, keep three things: photos taken before anything was moved, the company's moisture log, and a note of what caused the water and when you found it. Call the insurer early, but do not wait for an adjuster before drying starts: standard homeowner policies require you to take reasonable steps to stop further damage, and emergency extraction and drying are exactly that.
California has no separate restoration license. A company doing work worth $1,000 or more, or any work that needs a building permit, needs a state contractor license from the Contractors State License Board that covers the work. The record at cslb.ca.gov shows whether the license is active, whether it is bonded, and whether workers' compensation is on file. Ask which IICRC certifications the technicians hold.