The general idea is to build the transaction in a pipeline of function calls. Each function gets the old definition of the transaction as an argument and returns a new one. Eventually the call at the end of the pipeline executes the statements in the definition, wrapping them in a transaction.

If the question is how to deal with 100 or 100 k concurrent transactions, the answer again is in the same way a cluster of servers running programs written in any other language do. Eventually SQL is SQL.