File l3backend-pdfmode.def not found when loading expl3

For Windows:

1. Open the MikTeX Console
2. Check for Update
3. Updates > Update Now
4. Search l3backend package
5. Install the package

Since the July 1st, 2019 release of the LaTeX3 kernel, the driver-specific code contained in the l3backend module was moved to a dedicated package in CTAN, and now l3backend is a dependency of l3kernel.

The description of l3backend on CTAN says:

The func­tions here are de­fined dif­fer­ently de­pend­ing on the en­gine in use. As such, these are dis­tributed sep­a­rately from l3k­er­nel to al­low this code to be up­dated on an in­de­pen­dent sched­ule.

So if you updated l3kernel only without installing l3backend as well you'll get an error like this:

($TEXMFROOT/texmf-dist/tex/latex/l3kernel/expl3-code.tex)

! LaTeX Error: File `l3backend-<driver-name>.def' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: def)

Enter file name: 

and the solution is to install the missing l3backend package.