search for makefile pseudo-targets, and see an article, article link
if the author talks about pseudo-targets, it should be written like this:
.PHONY: clean
clean:
but in this example. PHONY: all is written after all. In this example, it is okay for us not to declare all as a pseudo-target? For example, I, make, default to make all, and then prog1 prog2 prog3 their respective dependencies to execute. So why declare all as PHONY, and whether there is a relationship before and after all:prog1 prog2 prog3?

is there any example to familiarize yourself with and use the usage of make?

-sharpsample Makefile 
all : prog1 prog2 prog3 
.PHONY : all 
prog1 : prog1.o utils.o 
cc -o prog1 prog1.o utils.o 
prog2 : prog2.o 
cc -o prog2 prog2.o 
prog3 : prog3.o sort.o utils.o 
cc -o prog3 prog3.o sort.o utils.o