A general, and disjointed overview (might not be acceptable in this sub):

Brittany -

The language is nearly dead due to a 400 year campaign by the Paris / Ile de France elite to suppress provincial languages (e.g. Occitan, Alsatian, Basque) and impose a cultural uniformity (Frenchness) on the lands comprising France. Ironic considering the huge sensitivity around maintaining French's integrity in the face of Anglo-Saxon influence.

Cornwall - the Cornish language is near death. Speakers are very low. It would take sustained investment by a government to even keep it as a recorded dead language.

The area of Cornish speakers has been gradually pushed further and further towards Penzance since the Dark Ages, due to proximity to Wessex, the most powerful and prosperous Saxon kingdom & later a bastion of the English nation state. The process was gradual because Cornwall was so poor, with little there besides tin mines.

Wales - Welsh is alive, but changing rapidly. While there are still many, myself included, who speak it with family members on a day to day basis, its use in daily life is being gradually replaced by it becoming an academic language, like Latin.

The program of Welsh medium comprehensives begun in the 1960s have ensured that a sizeable number (500,000) know how to speak it, but those kids will likely come from English speaking households and be more comfortable with speaking English outside of school.

Re. the history -

In medieval times, despite heavy Norman / Plantagenet occupation in the south, the nature of feudal society meant that common people naturally retained their language even as a ruling elite spoke French and later English. In the north, independent Welsh princes survived into the 14th century.

By the Tudor / Stuart era, Wales was formally annexed to England, but limited migration meant the language was intact, with monoglot poor people and a bilingual gentry. The culture of this gentry is fascinating - they appear to have conceived of themselves as both English participating in national scale political debate and institutions and Welsh, with distinct religious currents (especially crypto-catholicism, and later non-conformism and Quakerism) and with unique laws and traditions such as ty un-nos, by which a house built within one night is presumed to be legally yours, even if built on common land.

The change comes in the industrial era - Wales has a ton of coal, which was of course very valuable in the 19c. Mass migration from England to the South Wales valleys followed - Rhondda Valley's population went from 1000 in 1850 to 150,000 by 1890.

The migration was concentrated in the industrial South East of Wales - to this day, the old language is far more present in the rural west and north of the country.

Partly as a reaction to the flood of migration, the late 19c. saw a new Welsh nationalism determined to save the language, led by poets like Saunders Lewis and the recreation of the Eisteddfod - the ancient ceremony recognising the greatest bard in the land.

The efforts of Lewis and others academicised the language, marking it as being of literary merit (given recognition when Oxford installed a Regius Professor of Celtic Studies in the late 19c.).

From here a mostly middle class crusade started throughout the 20th century to save the old language - the creation of Plaid Cymru, the teaching of Welsh in school & the securing of a Welsh language channel, S4C.

Basically, the educated middle class in Wales forced the British establishment to recognise that there is another tongue spoken in the British Isles, and managed to do so before the native pool of speakers was completely swamped by the accelerated migration of the 19th and 20th centuries.

TL;DR - Brythonic Celtic is more or less dead outside Wales. It survived there because an educated middle class of Welsh speakers emerged there in the Early Modern Era who could both be Welsh and comfortably sit within British society. Their spiritual successors in the 19th and 20th century lobbied and organised to ensure the language's survival. Had they done so 2 generations earlier, I think Wales would be much more 'Welsh'. Had they done so 2 generations later, the last living Celtic culture would be dead.