If you are an experienced web designer, then you must be familiar with HTML and CSS. Bootstrap is simply a framework of pre-made CSS classes combined with some jquery to make it easy to build responsive websites without having to manually do all the flexbox legwork. It uses a large number of "helper classes" to serve as shortcuts to having to create CSS from scratch. (ex. applying the class mx-auto would horizontally center fixed-width block level content.) To get the most out of BSS, you don't really need to be sharp with HTML, because the program creates 99% of the HTML you'll need, but you do need to know you CSS.
Here's a "cheat sheet" that I used to refer to quite often before I memorized most of the classes I use often.
In terms of learning the Bootstrap Studio program and UI, I suggest watching the three tutorial videos, and simultaneously building the exact sites in those videos in BSS in real time, pausing the videos as necessary. This was the first thing I did when I got this program, and it really helped me understand the "Bootstrap Studio ethos" quickly.
Most software tries to be ergonomic in its own way... appealing and easy to use by most humans, but software developers can't step outside themselves when they're creating software, and use the program as others would use it, so they tend to sometimes take things for granted that end up baffling users. For example, for the longest time, there was (not sure if this has been corrected) no explanation in any of the BSS documentation on how to add a new page to a website. I figured it out pretty easily because I've been using computers for almost 40 years, and I've learned to just keep clicking shit until I find what I'm looking for, but I can't tell you how many new people have come to the BSS Forums asking "how do I add a page in BSS?"
The regulars who "patrol" these forums... Jo, marcco, twinstream, myself and a few others, have answered certain questions dozens and dozens of times to the point of exasperation, where we beg the developers, "PLEASE add this info into the tutorials somewhere, we're tired of explaining the same thing over and over to newbies!"
BSS is a FANTASTIC piece of software with a mediocre "instruction manual." I'd love to actually create a fully comprehensive user manual, but I run two full time companies, and I'm certainly not going to do it for free. I believe Jo has offered to help the devs create a true user manual as well, but they don't seem to want help from outside their "family", so we're all stuck with what we've got.
Heck, we can't even get them to add a new section to the forum. But when it comes to fixing bugs and updating and improving this software, the devs get 10 stars in my book. Take the time to learn the software and the U.I., and it will pay you back tenfold.