Apple laptops and iPads can handle 1gbps internet speed?

Yes, all of them support 802.11ac

MacBook 1.3GHz Ci5 (late 2017)

MacBook - Technical Specifications - Apple

Wi-Fi

802.11ac Wi-Fi wireless networking; IEEE 802.11a/b/g/n compatible

MacBook Pro

MacBook Pro (Retina, 13-inch, Late 2013) - Technical Specifications

Wi-Fi
802.11ac Wi‑Fi wireless networking;3 IEEE 802.11a/b/g/n compatible

iPad Pro

iPad Pro - Technical Specifications - Apple

Wi-Fi (802.11a/b/g/n/ac); dual band (2.4GHz and 5GHz); HT80 with MIMO

Depends on the exact models of laptop and iPad you are talking about. If you have somewhat recent devices they are specified for the newest tech and speeds. 802.11ac and Gigabit wired ethernet.

Whether you get that speed however, is up to many other environmental factors as well. Distance from router, obstructions like walls or large furniture, interference from other devices like cordless phones or microwaves etc... Actual speed delivered from your internet service provider etc...

Just because you change your router does not mean you magically get more speed. You would need an internet service plan that offers said speeds and then a setup that can successfully deliver that to devices.

Besides the capabilities per device, also consider the total: If several devices are used at the same time, their needed bandwidth adds up, quite literally. (For a car analogy: A 6-lane highway can handle more cars per minute than a 2-lane street, even for the same speed per car.)

I hooked up the faster internet today, my MacPro hard wired is getting close to 1gbps

The bandwidth (data link rate) of the connection and the actual data throughput speeds will be different. With the latter always being lower than the former. With Gigabit Ethernet, you should see data throughput within 90-95% of the bandwidth.

but my iPhone X, iPadPro, and MacBook are maxing out at 140mbps at either 2.4 or 5Ghz settings.

Wireless performance is getting closer to Gigabit Ethernet speeds but are typically considerably less. Each wireless device will be dependent on a number of factors, including such things as the number of spatial streams, and antennae that it can work with.

I am using an Apple Router/TC and extenders from the modem, but I think Apple Wifi devices cannot do more than the 140mbps.

... and here may be the "gotcha." When you extend a wireless router with another wireless repeater AND the connection between them is wireless, you typically half the bandwidth of your overall wireless network. To increase the overall bandwidth, you would need to use wired connections between base stations and create what is called a roaming network. Still not as "fast" as all wired connections, but significantly better than all wireless ones.

The half effect of the combination of wireless extenders is very interesting, which I was not aware of.

That's because bandwidth is required to maintain the extended network. This would be true for non-Apple routers as well. Mesh networking can overcome some of this by using a dedicated wireless backhaul for this ... but even Mesh cannot beat performance of either an all wired network connections or using an Ethernet backbone with wireless access points. The latter would be something like: Cisco AeroNet or Ubiquiti UniFi

Some mesh networks that utilize dedicated wireless backhaul are: Asus HiveSpot, D-Link Covr, Linksys Velop, & Netgear Orbi.

However..... when I disconnect the Apple Airport TC and even the 2 Apple Extremes (extenders), and just use the modem that has its own WiFi, I still get the same slower speeds inspite of being right next to the modem.

If that's the case, then adding the Apple equipment will not make it "better." There must be something amiss with your modem/wireless router/gateway device. Is the speed issue with wired or wireless or both connections using this gateway device alone?

I think the Apple devices are not capable of supporting anywhere near 1gbps speeds, except when hardwired where applicable

Not just Apple. Also remember that the newest 802.11ac version of the AirPorts came out over five years ago and have not had any significant updates since.

If you want to compare router performance, I suggest that you take a journey over to the SmallNetBuilder site where they compare numerous devices.

This is the guidance that Comcast puts out regarding their 400Mbps service: https://www.xfinity.com/support/articles/requirements-to-run-xfinity-internet-se rvice

At a minimum, you will have to list the exact models of your Macs and iPads.

https://discussions.apple.com/docs/DOC-5931

1 Gbps, not 1 gbps. G=Giga g=grams or the acceleration of gravity