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1 Yes, all versions of sketchup can open older versions. You can also use Save as to save a model back to an earlier version so you can still open a 2022 in 2019 for example.
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2 it’s not your only option, but you use the drop down menu of the download button in the browser version to select the Collada file. That file is basically a zip file that you extract to a location of your choice on your computer. (how all that works on a mac I don’t know) Once extracted you use File/Import in sketchup to import the .dae file that you have extracted. Done.