This can be either a cloud-based system or an On-Premise model. On-Premise models can be software-based installed on your existing hardware, e.g. a server or a VM.
With hardware modules you can extend the capabilities for your PBX appliance to increase connectivity, different lines, etc.
With extra software subscription you can add various features, e.g. hotel functionality or CMS integrations to your system.
A multitude of digital phones are available, from basic entry-level phones under $50 to video conference phones to over $800. Various suppliers provide differnt sets of features, most are interchangable.
or softphones, as an app on your smart phone or computer.Phones can be stationary as ma desk phone, cordless with DECT technology
In case you want or need to keep existing telephone lines like E1/T1/PRI or ISDN BRI lines you can either add such connectivity through harware modules (as described above) or add separate gateway appliances to your system. Or you can add single ATA devices to connect a single legacy device, e.g. a fax machine.
In any case you still need a phone number. This can be your existing one(s) or additional ones, maybe in different rate centers, or another country, or a toll free number. These numbers are then assigned to a SIP trunk, similar as it was assigned to a PSTN in the past .
There are many other options to concider, a horn or flasher for loud environments, adoor phone with or wwithout a camera, headsets, and many more.