Assuming Both sides that you want to connect have the same amount of vertices.
Step 1: select both edges like shown in image 1.
Step 2: Use "Bridge Edge Loops" (available in the edge menu (ctrl+e) or through the search function (F3 or space depending settings))
Step 3: Check for any problems
There is an add on included called F2. You need to go to Edit > Preferences >Add-ons and turn it on, with this you can select for example, 2 edges and press F to create a new polygon. Not sure if that's what you are looking for, but, here is an example (minute 1:29) :https://youtu.be/q1fEtys-RII?t=89