For "she" the sh becomes exaggerated "sshhhhee" as the tongue slides on the sides of teeth. Often termed a "lateral lisp". The "s" sounds flaccid and mushy but is still an "s". They might distort the sound so it sounds humorous but is still recognizable as the same sound. The other way to produce a sound incorrectly is to omit it entirely. "Book" would be pronounced "boo" or "toothpaste" would be "toopas".
Sounds can be classified in three ways: by place or where in the mouth they are produced, by manner or how they are produced, and by voice or whether the voice box is on or off. Often a professional, ENT or speech-language pathologist report may use reference to these terms of classification. Ask for explanation in regards to each aspect of the production of sounds. Besides those classifications a "phonological process" is an unusual decree that is being used and changes the place, manner, or voice of a group of sounds. Some phonological processes are: fronting, backing, gliding, cluster reduction, devoicing, stopping.
Adults and children can have articulation challenges. Children who do not receive speech therapy and do not "outgrow" their speech difficulties will often continue to make errors when speaking as adults. Why is this? The patterns used for speaking are reinforced with each subsequent utterance if new ways to produce the speech errors are not learned. Often some error sounds are termed "developmental errors", the ongoing reflection is whether a child will outgrow these patterns of speech or require therapy to undo the incorrect sound production.