Textjoin function based on changing row number

I am trying to create a text join from this raw data source.

I want to create a 'description' field in another sheet that 'textjoin' the description from Column G.

So it needs to textjoin G2 - G8 for the first title, then based on where the second title is in column B, Text join G9 - G15.

This is hard because there is a different number of rows for each description based off the title in column b.

Is there a way to textjoin rows with key BTRTN-1573 or blank, until BTRTN-1569, then start new text join in the row below?

So . the description would be for the first one: "As a member of the team, I want to do something, So that I can do something else, Success criteria:, Completion of the formula, No dependence on others"
And the the second one: "As a member of another team, I want to do something else, So that I can do something again, Success criteria:, Completion of a different formula, No independence on others"

Currently all I can figure out is how to do one at a time with this formula:
=TEXTJOIN(",",TRUE,G2:G8)

Thinking something like:
=IF(B2="",=TEXTJOIN(",",TRUE,G2:G8),"")

Attach a sample workbook (not a picture or pasted copy). Make sure there is just enough data to demonstrate your need. Include a BEFORE sheet and an AFTER sheet in the workbook if needed to show the process you're trying to complete or automate. Make sure your desired results are shown, mock them up manually if necessary.

Remember to desensitize the data.

Click on GO ADVANCED and then scroll down to Manage Attachments to open the upload window.

Attached - As I was doing this realised that there are a few other things that are going to be hard.

As the raw data source is updated with the description rows changing each time for the 'key', how will the second sheet stay up to date?
Is there a way to set the formula so it is searching for a new key then picks up the required description for just that key?

Amazing! That works the way I was hoping, but how would this formula be continued down the page? I tried to copy the formula on the 'after- output data' page onto rows below with new data on the 'raw data' page and it didn't work.

The raw data sheet I actually have is 400 rows of raw data and should go into around 100 rows on the new sheet.

Figured it out - thanks for your help