How change currency sign from US$ to only $ (Not Region in US)

You can create your own custom formats. Here is one for US Dollar using "accounting style". I don't like negative signs when using currency, they are easily overlooked.

The $ symbol is not the one of the tokens from below, it is a typed in $.

The -- is the space token, set to "flexible"

If you do not want accounting style, don't create rule 1 and remove the space from the default format.

For Thai Baht, make another of these but replace the $ with ฿

Start a new blank document, make your custom formats in it (see below), save it as a template, and use that template for new documents (set it as the template to use for new documents in Numbers/Preferences).

You can copy a formatted cell from your current document and paste it into the blank document to transfer the format to it. And, if in the future you create a document using some other template, you can do the same thing to get the format into that document if you need it.