To get an overview of Script and its corresponding opcodes, take a look at https://en.bitcoin.it/wiki/Script

For an easy execution environment, check out the online ide scriptwiz (https://ide.scriptwiz.app/) made by Burak

btcdeb (https://github.com/bitcoin-core/btcdeb) is also a good way to get a better idea of how opcodes manipulate the stack

From there I'd check out miniscript, which is still far from having a friendly developer environment but maybe worth learning about in the longterm. Here's a good video to learn the basics of what it is

For applications, the Liquid ecosystem has some good examples. Check out the Fuji whitepaper for an example of complex scripting (https://vulpem.com/synthetic-asset-smart-contract.pdf) along with Liquid's non-custodial options (https://blockstream.com/assets/downloads/pdf/options-whitepaper.pdf)