From memory, I believe Leavers claimed there would be more jobs in sectors where jobs were being done by immigrants eg fruit picking, care work, packaging, courier work.
Remainers claimed (and they were right) that there would be a loss of jobs in financial services and the car industry.
The first jobs I remember going were the 2000 from the European Medicines Agency when it transferred from Canary Wharf to Amsterdam. I remember that one because a friend was made redundant.
So the net result would be loss of high paying jobs, to be replaced by low paying ones.
The loss of retail jobs probably doesn't have much to do with Brexit, but the high street was on its last legs anyway and Covid-19 has hastened its demise.