How to type letters when sending text messages?

Problem description:

Sometimes I have to type letters when sending text messages, but after I choose letters, because a key has three letters, the initial letter (such as A) can be read directly, but what if it is a non-initial letter such as B?

Analysis:

Generally, it is continuously pressed. For example, press the "2" key once "A" and press the "B" key twice. .....