Syntorial (on desktop or iPad) is an app which teaches you synthesis by doing - it’s a full synth which you can use in its own right, and it introduces you to each part and gives you challenges to recreate a certain sound using that part. I can’t say I’ve done the full thing but it’s a cool concept.
You can also just download some free/cheap synths (or get a free trial of Ableton or some other music making software which comes with its own included synths) and start messing around of course haha, there are a tonne of good ones around, including on iOS if you use an iPad (the advantage being the touch screen makes it maybe more fun than using a mouse). GarageBand on Mac is maybe a fun place to start experimenting too.
Happy to advise more if you have specific things you’re interested in!