Digits grouping / thousands separator

How do I enable/disable digits grouping? How to I change the separator to other character?

Look up NumberForm in the help system and in that click on Details and Options.

thanks, but I’m not asking about output. I want digits grouping in for example title, paragraph etc
and I need disable digits grouping in expression made by typesetting

I have this feeling that no answer is going to be acceptable.

Try creating an ordinary input cell containing

NumberForm[N[Pi, 12], DigitBlock -> 3, NumberSeparator -> "_"]

Evaluate that.

Then create a title cell with contents, but without the number you wish to display.

Scrape the output from the NumberForm evaluation.

Paste that into the appropriate position in the title cell.

If quotes appear around that paste then try to remove them.

When I try to directly evaluate in place a NumberForm expression within a title cell this does not seem to work.

When I enter long strings of digits into a text cell there is no default automatic reformatting of digit blocks.