Being selected for a county team may be as simple as letting the captain know you are available for selection, for many captains discover that a large part of their job is twisting arms to get a full team out.
Some county boundaries seem to be policed more strictly than others. For example Hinckley play in the Leicestershire league, their nearest neighbours are Nuneaton 3 miles away in Warwickshire and who play in the Coventry and Leamington leagues. Lowestoft (Suffolk) play only in the Norfolk league, whilst Northampton play in both the Northamptonshire Silver Kings league and the Bedfordshire League. Examples like these abound.
The county structure in chess was established well before the late 20th century administrative reorganisations and so is based on the traditional counties. However there are now both Merseyside and Cleveland associations. To add a little confusion to Chingland the maps show a mixture of pre- and post-reorganisation boundaries.
Yorkshire on the other hand does have a county league as well as numerous district leagues based around its many sizeable towns and cities.