Hello friends. Is it possible to install Mac system applications on external memory?

You can copy them there, but then you'd just have them in two places since you cannot in any way remove them from the startup drive.

Also, some apps simply will not run well unless they are on the startup volume. And, by the way, an external drive is not "memory" - it is an SSD or spinning drive that has a certain amount of space to use either as a backup or, if compatible, as a startup drive. Memory is your built in RAM chips.