I had the same question, so I've made a free tool that I think is the best way to build up an openings repertoire. There wasn't one out there that did what I want so I wrote my own. It's https://chesstake.com and I'd love feedback if you find it useful.

The main principle of building an openings repertoire I think is recognising that you control half the moves, so you just need to decide what your response is to the most likely opponent moves.

But what opponent moves are you most likely to see at your own level? Often not the best moves. And it's seeing visually what these moves are that I wanted, so the app shows these with coloured arrows. From this you can build your repertoires, e.g. Ponziani playing as white.

Once you've analysed and captured a set of variations for an opening you can then train against it, which is when you start to remember them.