A participant can join the network with as little as one plot, which the minimum size for Chia is k=32, or ~101.3GiB (~108.8GB). Farmers with a small amount of capacity, and therefore an extremely low chance to win a block reward, will most often choose to join a pool.
In Chia the pooling protocol is all done on-chain and the farmers still create the blocks, which maintains decentralization. The only resources required to start are enough free space for a single plot and database and a computer to host the full node software. There is optional farming software from 3rd parties that the consumer only runs a light client, running either only the harvester or the harvester and the farmer and connecting to a remote full node. This eliminates the requirements to run a full node, but most farmers are still running a full node due to the low requirements and independence and decentralization benefit. This is also the default mode in the official Chia wallet and farming desktop application.