The SketchUp 3D warehouse no longer converts models to the 17 or 18 format, only the last three years versions files are created for each model. You can no longer load models directly into your model space using the 3D warehouse button in SketchUp 18 or earlier. You can find some models available in the component browser search function of the 3D warehouse from within SketchUp. Window>Components> type your search into the field there, to download a model just click on the model icon. You can still get all models, you can visit the Warehouse in a separate browser and download the COLLADA (.dae) version of any model you find there, then import that into your SketchUp file.