Inspiron 15 5000 infinite boot loop

In case of Infinite Boot Loop, following are the may be the reasons. 

#1. Run a Diagnostic Test: As you said that you're not getting any error while running the diagnostics test and everything is working perfectly, especially the HDD, and RAM. I'm not gonna discuss this part as this is not the problem as per your statement. 

#2. BIOS Setting: If you've done everything on your side in BIOS, try switching between the legacy and UEFI modes.  

#3. If after the hard-drive check, it's good and after you checked BIOS but it's still not booting up correctly, you're still in the boot loop, you're still stuck on the loading screen, it's most likely the third option which is an operating system error. It may be corrupt. Try factory reset. If you don't know how to do, simply search on YouTube. If your operating system is so corrupt that a factory reset will not process, then the last option is reinstalling your operating system from scratch. 

After you've gone through all these options, don't worry, you'll be able to fix this error either one of those three things, your hard-drive, your BIOS settings or your operating system.

https://www.dell.com/support/article/en-us/sln297920/reset-or-reinstall-windows-10-on-your-dell-computer?lang=en

I have googled on how to factory reset my laptop but I get multiple results with different guides. Do you know how to properly do it??

Software conflicts may occur when you install a program or an update or when you run a program in Windows 10, thus causing boot error. You can determine what conflict is causing the problem by performing a clean boot. Follow the instructions below to troubleshoot Dell boot loop:

1. Boot into Safe Mode.

1.1. Power on and off your computer three times. On the third time, your computer will automatically boot into the Windows Recovery Environment (WinRE).

1.2. Select Troubleshoot > Advanced Options > Advanced Startup Options > Startup Settings > Restart successively.

1.3. The computer will restart and enter the Startup Settings menu. Press 4 to select Enable Safe Mode.

2. Perform clean boot.

2.1. Tap the Search button on the taskbar and type msconfig. Select System Configuration from the search results.

2.2. On the Services tab of the System Configuration dialog box, check Hide all Microsoft services and then click Disable all.

2.3. On the Startup tab of the System Configuration dialog box, click Open Task Manager.

2.4. On the Startup tab in Task Manager, for each startup item, select the item and then click Disable. Close Task Manager.

2.5. On the Startup tab of the System Configuration dialog box, click OK, and then restart the computer.

3. Determine the problem item.

3.1. If the boot loop does not occur in a clean boot environment, then you can determine which startup application or service is causing the problem by systemically turning them on or off in the System Configuration dialog box and restarting the computer.

Tips: The most efficient way to do this is to test half of them at a time, thus eliminating half of the items as the potential cause with each reboot of the computer. Repeat this process until you’ve isolated the problem.

3.2. Once the problem item is found, contact the program manufacturer to determine whether the problem can be resolved. Or, you can run Windows with the problem item disabled.

4. Reset the computer to start normally.

4.1. Enter System Configuration as mentioned above.

4.2. On the General tab, click the Normal Startup option.

4.3. On the Services tab, uncheck Hide all Microsoft services and then click Enable all.

4.4. On the Startup tab, click Open Task Manager and then enable all of your startup programs. Close Task Manager and click OK on the Startup tab of the System Configuration dialog box.

4.5. Click Restart when prompted.

I’ve tried to boot into safe mode this morning, I restarted it a multitude of times, but nothing happened