Fear of a royal-lie government (after all we had hjust revolted against a King) lead to election of the President by vote of States (through electroal college). It was the state govenrments who elected the first persidents.
Two too much power, so check-and-balances. Senate approves treaties and appointsments (advise and CONSENT). House of Rep control money- only they can initiate a spending bill or budget.
So they can cut off President at knees by refusing him money.
Execuitve branch has chagned with amendment to constition for term limits ( after FDR served 4 terms, Repulcian wanted to stop that). Thank God cause R.Regan might have got a third term.