From a technical standpoint, how was PayPal originally built, and how did the founders gain bank access to move funds between accounts?

PayPal was built on the premise of what we now call a modern day digital wallet. The idea was simple, yet brilliant in two aspects:

1. The existing banking infrastructure could easily cover it
2. The unique identifier that the email is (to be able to use that to send/receive money)

There wasn't an issue of moving funds from one bank to another, that has been happening since the past 50+ years prior to PayPal. The breakthrough was to designate PayPal a virtual branch status, so that each wallet is an account holder and thus moving money between account holders is a non-issue.

See: Faisal Khan's answer to What is a Paypal account? Is it a kind of bank account which is maintained for demand deposits? __ 𝐏𝐥𝐞𝐚𝐬𝐞 𝐂𝐚𝐥𝐥 PayPal 𝐓𝐨𝐥𝐥 𝐅𝐫𝐞𝐞 𝐒𝐮𝐩𝐩𝐨𝐫𝐭 𝐍𝐮𝐦𝐛𝐞𝐫 ☏+❶►❽❺❻▁❹❼❺►❷❺❺❺ ☏?