@Brefugee - parkrun all started when the founder organised a 5km time trial at Bushy Park for some friends and club-mates when he was injured, and he still wanted to be involved with running. That became a regular thing, and then some other events were set up as off-shoots, but parkrun has moved on a lot since then. It isn't a race, it's just "a run" or even a walk - they actively want to set up new events in areas where people wouldn't think to take part in an organised physical event. They've changed things like the "tail runner" to the "tail walker", and celebrate that the average finish time is getting slower as it means there's more people walking or walk-running rather than speedy club runners!
But yes there is a central organising HQ with a small number of paid employees, however all the actual runs are put on by volunteers. If you want to start a parkrun, you map out a plan and then contact HQ who help you with liaising with land owners and signing off risk assessments and things like that. But you need a dozen "core team" volunteers to be the event directors and run directors who cart the kit (hi-viz, cones, tape and posts to make a finish funnel, megaphone!) around each week, manage the volunteer marshals, process the results so people get their times, deal with any incidents like injured runners or complaints or sorting things out if there's another event on in the park so you have to cancel one week. We sort it all out week-in, week-out, we only contact HQ to renew the risk assessment each year and if they need to follow up on a serious incident like someone getting injured during the run.
The HQ side of things arrange sponsors from companies who help to fund things like the website that hosts parkrun event details and means you get your results emailed to you, they promote parkrun as a brand. Which is all over the world - you can go and do a parkrun in Australia or South Africa! And yes you can pay for a t-shirt to show you've run 50 parkruns, and that spreads the word of parkrun. But the events are run by thousands of volunteers each week. We do it because we believe in it as a nice community event, it got me into running, and I've made lots of friends doing it.