I don’t think there is a lot of tech in this game when it comes to driving, therefore it is pretty easy to get decent quickly. Many time-savers however are mainly learned by experience, like on some random spots on the track it is faster to start your drift than others (sometimes it even feels like you get a slight boost forward). Live videos by good players are therefore helpful because you can easilier identify these spots.
The combos are on the WR site, if you are interested in online use metal mario with B-dasher/Blue 7/Bruiser with (red) monster wheels. I made this video
https://youtu.be/gc8PA-e1fT8 but it mainly demonstrates wifi lines. In some parts I do have some explanation, like the part on TC might help with TTs as well.
If you want to get good quickly I suggest you get good at one specific track first, that is neither too difficult (like rDKP or rKC) nor too different from the rest (like WS). My suggestion would be to try and get (as close as you can to) sub 02:04 on SGB without using the stall boost. The way I suggest you to learn is, watch the WR (for sgb I would suggest the best no stallboost run you can find in the WR history), and then get a PR. Then play the WR video again right next to your PR’s replay, both in first-person mode. When you move your steer and the WR does not, or the WR does something you do not do, assume that you did something wrong there and in your next run, race against your ghost and focus on doing these parts just as the WR does them. If you keep repeating this process you will get good quickly. It’s imo a better way to start learning than watching live videos, I think you can best learn from these when you’re already good or you’re struggling to understand a certain strategy.