You e-sign the investment documents when you check the terms and provisions and click the commit button.
Your e-signature is in lieu of a physical signature on the subscription documents and satisfies the requirements of the U.S. Federal ESIGN Act of 2000 and the Uniform Electronic Transactions Act.
Once the deal is fully closed, official AngelList signed docs are shared with each LP. Your signature will not appear in these documents, but your e-signature certificate along with these investment documents comprises your signed deal documents.
A copy of the investment documents, and your e-signature certificate, can be found by navigating to your portfolio page, selecting the relevant investment, and clicking "Final Investment Documents" or "Signature Certificate." You can find more detailed instructions here.