This free design tool is published and maintained by BrickLink, the biggest secondary market for LEGO pieces (see below). BrickLink StudioīrickLink Studio, sometimes simply referred to as “Studio” or “Stud.io,” is the best option for most builders of intermediate to advanced skill. There are three notable options at the time of writing. The easiest way to do that (without spending a mortgage’s worth of money on randomized pieces) is to use a software tool to build a set virtually. In addition, we're going to upgrade the Studio software every month to improve the user experience and enhance it's performance.If you want to make a LEGO set with hundreds or thousands of pieces, you want to plan it out first. They also released a video last December (2020) where they updated on the future of both BrickLink and Studio. The LEGO Group is planning to support and evolve Studio and will review opportunities for both services following the closing of the deal. Whilst we can't say exactly what The LEGO Group will do in the future, they did say when they acquired BrickLink that:

I'm not aware of people adding more elements to the official library files, but I've not really looked for it. Glad you've managed to get it working in your preferred version. However The LEGO Group have purchased the team behind Studio, which is actively developed and has new parts regularly added.

LEGO Digital Designer is no longer actively supported.

To see all the parts, you may need to enable 'Extended Mode' (View | New themes | extended).

However you can use the parts library ( Assets.lif) from 4.3.11 with 4.3.12 - this can be extracted from the installer if you open it with an archive explorer (i.e. The last release, 4.3.12 has a known issue with missing parts.

I believe that the last version that had a complete set of elements was 4.3.11, however that has dependencies on Adobe Flash, so will likely not work on most modern systems that have now removed Flash.

Lego digital builder