[Nexus 7 Q&A, Help & Troubleshooting] flash713:
User who commented just before me is right. It's also possible that you may have done something wrong who knows, it happens... When you search for information about Android always make sure you know the source at least somewhat and that they're legit. I have a circle of developers I can go to and ask questions to about whatever and I've known them for many years. It's like reading books. We must be careful what we read because there are people who are crazy , lots of them and you never really know unless you know them. It's sad to say but there 's TONS of misinformation out there about Android!! Trust XDA but idk about many others out there.. I always end a search phrase with "XDA" and it makes things easier to find here sometimes. Check out some of the telegram Android device custom rom development groups. I go there just like I go here to XDA to read and search and help someone if possible along the way. :)

So you cannot hold down power+ volume + and - until you see bootloader mode? If you can try that. You should see a black screen and "START" you can use volume +&- to navigate through various options.. Press power button once if you can get to this screen. Devices should reboot system. If it doesn't and it only returns back to where you are now then I suggest flashing a factory image and formatting data. Does your computer register that your Nexus 7 is connected in your device manager when you plug it in with a USB cable? If it does I would flash a factory image using fastboot commands and make sure that you format your data at the end of the flash. You know: sudo fastboot -w ,that will wipe the device. If you are still able to send fastboot commands then you are okay. Flashing a factory image should work...

I haven't had a Nexus 7 since about 2014 so I don't remember much from that device and what is available for it but I would search here for maybe some sort of unbrick tool to get it going again. I'm not sure anything exists like that for it but if there is it's here on XDA over in the Nexus 7 section of the forum. Link for it is below. Do you have the link to 5he video you followed when this happened? That would be helpful and also a detailed list of exactly everything you've done, in precise detail before this occured. ..

Nexus 7 section of XDA:

https://xdaforums.com/c/nexus-7.1673/

Tiny adb and fastboot tools for Windows: (This is updated and current. I wasn't aware that the old 15 second adb fastboot installer was no longer being updated so I returned to edit this post and recommend another similar installer that is currently being maintained and updated.. Its been many years since I've used Windows ..

https://xdaforums.com/t/tool-windows-tiny-adb-fastboot-april-2021.3944288/

Factory images for Nexus 7: https://developers.google.com/android/images

Hopefully this helps a little. I assume you use Windows. I switched to Linux many years ago but that Installer is really easy to do and literally takes 15 seconds. Not having fastboot and adb on your PC and messing with Android like this is nuts! This is just my suggestion.

Edited post 05/21/21. Deleted info about 15 sec adb fastboot installer and replaced with "Tiny adb and fastboot tools" link, which is up to date unlike the old 15 sec installer...