BSS function… that’s a very large topic.
BSS includes:
equipments : MS, BTS, BSC, MFS, TC
interfaces : Um, Abis, Ater CS & Ater PS.
I recommend you focus on one equipment only, such as BSC or BTS. Learn commisioning, installation, configuration, supervision, maintenance… That’s a job you cn do for about 5 to 10 years.