In economics, traditionally 'land' is any fixed resource:
"In economics, land comprises all naturally occurring resources whose supply is inherently fixed. Examples are any and all particular geographical locations, mineral deposits, forests, fish stocks, atmospheric quality, geostationary orbits, and portions of the electromagnetic spectrum."
(Wikipedia, "Land (Economics)").
Land in the literal sense, as in ground area in cities, is also in a sense fixed, but also may be said to have an ongoing supply, due to various factors. As a first effort at cataloguing those:
1. reclassifying - rezoning; changing urban growth boundary; releasing for development (eg council land); land reform.
2. eminent domain, compulsory purchase, expropriation.
3. technological change, including making new land effectively available by new transport and making land more productive by increasing built volume per unit area.
4. reclaiming - demolishing/redeveloping, remediating brownfields, geo-engineering.
5. market shifts? regulatory or geopolitical change causes major structural shift, thereby making land effectively available to new parties on a large scale. For example, large-scale inflation or deflation, ban on foreign ownership of property, capital flight, government insolvency.
"Land supply" is not merely an abstraction, in that it is concretely measured, planned, and regulated in many contexts.
For example, in the UK "5YHLS" or Five Year Housing Land Supply is a government policy requiring COuncils to demonstrate the availablility of developable sites to fulfill five years of forecasted housing need.
The analyst James Gleeson has presented astonishing official data on how land with permissions for homes to exist has become radically more expensive in the UK over the last 60 years.
In California, #RHNA Regional Housing Needs Allocation requires similar of all cities, tho is kind of sham & in YIMBY crosshairs for reform.