For the best result in your UI design projects, I’d suggest using a vector illustration software. These programs use mathematic equations and geometrics to create really clean, scalable images - perfect for UI! 
The best free option out there is probably Inkscape. It’s a really handy vector illustration software and you can accomplish quite a lot in it. I actually used it to create the UI in some of my earlier projects. There’s a bit of a learning curve and it takes a while to get used to but once you get the hang of it, it’s really versatile. If you’d like to learn the basic, you can find a good tutorial playlist here!
If you feel like making a once-off purchase for a more professional software with a few extra features and a cleaner interface, I’d highly recommend Affinity Designer. It’s been my vector illustration software of choice for the past few years. As @wevetments has already touched on, its used in many workplaces and is an industry-standard tool. It doesn’t have quite as much functionality as Adobe Illustrator (text manipulation is particularly difficult) but for the most part, you can create whatever you want in it.
I hope you find these suggestions helpful!