@entozoon please don't. You can edit your first post and make it clear it's a Raspbian issue preferably leaving original text in place for reference) (please don't blank your post as it will mess the thread up). You can't delete the entire topic anyway.
@MDtar7 Im running out of ideas, but try enabling SMB 1.0 support on your PC.
Its in Control Panel, Programs and Features, Turn Windows Feature On or Off and select SMB 1.0/CIFS File Sharing Support. Restart your computer and try it again.
Screenshot from 2020-03-26 12-21-15.png
@dandy_j Can you run sudo rfkill unblock all (from the terminal, after exiting Emulationstation) and see if you can detect the wifi networks afterwards.
Is your WiFi configured for 2.4 Ghz b/g or it's just 5Ghz ? The PI only has 2.4 Ghz capabilities.
Also I can't seem to SSH from my PC, regardless of what IP I use.
Well, if there is no network configured it's not possible to use SSH to the PI, unless you have a wired connection. If you have one, then you first need to enable SSH, since it's disabled by default - use the raspi-config script (see here).
Remember that the PI 3 only connects over 2.4Ghz, so if your access point only works in 5Ghz, then you'll not see the SSID. This happened to me the other day. Maybe the external wireless adapter has 5Ghz support and that's why it's able to connect.
Here is a Wireless configuration guide for the PI - including command to scan for SSIDs and configure the wireless from command line.
All that I can suggest is to connect another device to the network port of the PC to see if the network connection is shared to that device correctly.
Then at least you know if the Pi or PC is to blame.
personally, when things like this take too long I give up, in this case I would just conenct the Pi directly to the router.
The power supply is 2Amp, is it weak for the WiFi to function properly?
@hellska It probably depends on what else you have attached to you RPi, but the specs for RPi3 say 2.5A is recommended.
Your issue reminds me the behavior of my RPi1 with an external Wifi dongle. I never confirmed it was caused by insufficient power, but the fact was I had a poor 1.5A power supply back then. Using scp (which is consuming bit of CPU btw) was causing my Wifi to disconnect many times.