It should be what it your ISP is claiming. However, my fiber ISP offers several tiers above my 500 Mbps rating, specifically, 1Gbps and 2.5Gbps, and they do state specific technical requirements for that to work. I have a 1-Gbps wired network in my home, so in theory, my home network could support up to a 1-Gbps internet connection.
I'd say that it is going to be dependent on what WiFi standard is supported by your router and what standard is supported by your network card. Here is a link that lists some Wi-Fi standards and what they are capable of
https://www.speedguide.net/faq/what-is-the-actual-real-life-speed-of-wireless-374
I will say that I have a Moto X4 phone that supports 802.11 ac and an Archer TP Link AC1750 router that I use as a WiFi hot spot
https://www.tp-link.com/us/home-networking/dsl-gateway/archer-d7/ which also supports 802.11 ac. I have not speed tested the phone, but the WiFi connection between the router and my phone shows a status of 500Mbps when it is connected. In theory, I can get no more than that for internet speed since that is the speed of my internet connection. That speed, however, is only because there are multiple, simultaneous WiFi channels bonded to act as one between my phone and my Hot Spot. From the page I linked
So, I'd say do some research on your WiFi router. Finding out its maximum speed will give you an idea of what to expect between your WiFi router and your WiFi NIC. After that, your NIC will have to support the same standards as your router and it looks like MIMO in addition to the base standards that your router supports to get anywhere near 1Gbps.
I chose a wired network for my home for other reasons, but it turns out that there are, to me, many layers of technical complexity with WiFi so that in retrospect, the wired network was a better and simpler choice than WiFi even though I had to run wires through walls and the other complexities physical complexities of running wires.
BTW - I don't consider myself an expert, so I advise researching this on your own.
However, to get back to your original question, if your ISP says that the connection is 1Gbps, then that is what it should be no matter what - the caveat is that your router/home network must be capable of handing that speed as well.
One way of testing this might be to connect a PC/laptop with a 1000Base-T wired NIC (I.e., 1Gbit wired nic) directly to your ISP's modem and run a speed test through something like
www.dslreports.com That will help to narrow down the possibility of where the bottleneck is.