Mulch and compost are the default treatment in a garden, not fertilizer. They feed the soil with decomposing organic material, and the soil life feeds the plant steadily, on its own schedule, at no further cost to you.
Organic matter goes on top, as mulch. The soil life takes it down at its own rate, which is the rate the plant can use it.
How deep
Three to four inches. Once a year, refresh it: spread half an inch of compost over the mulch and rake it so it sifts down, then top the mulch back up to three to four inches.
In sandy soil, mulch heavier, because it breaks down faster.
A doughnut, never a volcano
Keep mulch several inches back from trunks and stems so they have room to breathe. On a mature tree, keep the root flare clear. The doughnut is a ring with the trunk standing clear in the middle; the volcano is mulch heaped up against it.
When planting, push the mulch aside so it doesn't fall in the hole. Wood chips do not rob the soil of nitrogen unless they are buried in it.
What kind
Arborist chips or your own chippings are the best. If you need to bring in mulch, use coarse to medium, around three quarters of an inch. Do not use fine mulch; it packs down, stays wet and grows weeds in the mulch itself.
Never use dyed mulch. It is usually milled from recycled construction wood rather than from trees, so it may contain contaminants, and the dye can leach and stain hardscape.
Never use rubber. It does nothing to feed the soil, and it heats up, smells, and looks unnatural.
Some plants get different treatment. California natives, especially manzanita, ceanothus and oaks, and proteas, grevilleas, banksias and other Australian and South African natives get no compost and none of the smaller mulches. Use coarse bark or gravel as mulch for these.
Make your own
Run what you prune through a chipper and put it back where it came from. There is no landfill, no hauling and no dump fee, and the material you would have paid to get rid of replaces material you would have paid to buy. It counts twice. The exceptions are diseased or infested material and flowering weeds.
Leaves work the same way. Blow them into the planting beds so they tuck under the shrubs and feed the soil. That means free mulch, no waste, and no hauling. The exception is diseased leaves. Like all infected material, they go straight to the trash, because leaving them re-infects the plant every spring.
Near the house in fire country
In high fire zones, no bark mulch or wood chips within the first five feet of the house. Use rock or decomposed granite there instead. Chipped prunings used as mulch can stay everywhere else.
The full rule, with its illustrations, is in Southern California Landscape Maintenance: Ecological Maintenance, Simplified, at rootandbranchdesign.com/book. The first chapters are free to read at rootandbranchdesign.com/book/sample.
The material you would have paid to get rid of replaces material you would have paid to buy. It counts twice.