Today I bought the Sims digital deluxe as a gift for my mom and sibs& what even comes with base game

Not sure what you mean by free stuff when you signed up first.

If you got the deluxe edition you will also have the tiki bar, animal hats and a few outfits that come with it.
link to the items in the delux upgrade

The holiday stuff is available as a free DLC. It is not added automatically, you have to click to install it. At least that is what I had to do and I started sims 4 when it was around for a while. You can find it when you open origin, go to the sims 4 in your library and the select extra content. Alternatively in the start up screen in game you can go to packs and then free stuff.

Grims guitar is also available when you look in the start menu in the game under packs and free stuff. I haven't tried it yet but I expect it to work. Why else would it be listed.

The only thing that I know you can't get anymore is the picture frame with the family portrait. That one was only available at the launch of sims online.

The mood lights from sims 3, the plumbob lights, will only be available for sims 3 owners.

And of course if your mom & sibs patch up their game, they will get everything that was patched in for the base game over the years. For example I started after the toddler patch but immediately had pools, ghosts, the athletic career and all the other stuff from previous updates.

edit to add link