Jack, we have been enrolled in the UK Biobank for a good few years now: apart from one face-to-face interview and a few health checks some years back, it has mainly been based on online questionnaires re our state of health, weight, height, diet, lifestyle, etc. The Covid antibody test was offered as the latest intervention, so we both signed up for it: a kit was sent in the post, with full instructions how to do your own test, which involved a finger stab with provided lancet and a device into which we had to put a drop of blood and supplied reagent, and read the result 20 minutes later: then just send the result back via the website.
See: UK Biobank