Reindexing is a node/blockchain operation. It rebuilds the block index, and re-validates every transaction in the blockchain. It can be done in Bitcoin Core (including version 28.0) using the -reindex command-line option. Depending on the specifications of your machine, this may take hours to several days.
Rescanning is a wallet operation. It goes over all blockchain transactions again, seeing if they may be relevant to your wallet. It can be done in Bitcoin Core (including version 28.0) using the rescanblockchain RPC command (in the GUI, one can use the debug console to give RPC commands). The -rescan command-line option was removed in 23.0: https://bitcoincore.org/en/releases/23.0/
