The origin of Mother’s Day goes back to when many people were employed in service for ‘the big houses’ ie the very wealthy. It was usually a live-in role pretty much 7 days a week, and often far from ‘home’. At this time the UK was predominately a church going, Christian country - churches were packed every week. People mostly lived in the same small area they’d always been, with generations of the same family all being close by.
Once a year, (and I think something to do with the Christian church calendar, and X weeks from Easter or something) people got the day off from ‘service’ to go home to attend their ‘mother church’ - hence, Mother’s Day - the church they attended as a family in the area where they’d lived/been brought up and would be baptised, married, buried. Each woman was given a little bunch of flowers at this service (usually local daffodils) and this tradition continues at come churches now. As they were ‘home’ they’d spend time with their family, including the mum and might even have said ‘happy mother’s day’ meaning ‘happy day of getting back to your mother church’
But that’s all it started out as. A cherished and welcome, rare, day off to go home and attend your ‘mother’ church.
Originally it was nothing at all to do with actual mothers - card manufacturers took over many decades ago and pushed it, and now shops/retailers/restaurants are pushing it even further to encourage people to spend and make lots of people feel guilty if they don’t do anything for their own mother. Or cause huge family arguments if they don’t want to see the MiL, or want to have the day with their children and not their mum etc etc. Or feel down and despondent because they’re not a mother, can’t have children, are NC etc.
It’s essentially now another big marketing ploy and will get bigger and bigger as the years go on, but started as something quite nice and simple.