There are ten hotbars available to use but as the default setting, Hotbars 1-3 are the only ones that change between Classes/Jobs. Hotbars 4-10 remain consistent between Class/Job changes. You can change this! Go to Character Configuration, then the Hotbars, and then the Shared tab. The unshared hotbars will be tied to that specific Jobs (like what I have for hotbars 1-3 below) while shared hotbars will be shared for all Jobs (hotbars 4-10).
You'll want to keep your Class/Job specific actions on unshared hotbars as opposed to shared hotbars.
The default keybinds are 1-= on the keyboard, but due to the sheer length, it is highly advisable to shorten the usage of the Number row and use other keys around WASD.
You can change your keybinds in Settings --> Keybind --> Hotbar.
I personally use 1-6 and the letters Q, E, Z, X, C, and F for my keyboard. My friends use 1-5 and Q, E, Z, X, C, R, and F. Select a range that is both feasible and comfortable to use. Arrange yours accordingly to your keyboard and what makes sense. The idea is to pick buttons that you can easily press with your left hand.
To keep from having to use individual keys for every action, you can use combinations of keys. You can use the Shift, Ctrl, and Alt keys as modifiers. Note the little Up Arrow, C, and A icons next to the key names to know which modifier is for which slot.
You can also use mouse buttons for actions to split things up between your hands! You can also apply the Shift/Ctrl/Alt key modifiers as well.
You might have noticed that there are the [, ], and \ keys on my Mouse Hotbar. This is because the game only recognizes up to 5 mouse buttons, Left click, Right click, Scroll Wheel click, Forward, and Back on its own. I have the Logitech g502 Hero mouse and you can map unused keys to the additional mouse buttons and then map the corresponding keys to the hotbar.
I put miscellaneous items on my shared Hotbars ranging from Emotes, to markers, gear sets, food, Ready Check, Countdown, Mounts, Aether Currents, and toggles for other Hotbars and just click on them since I hardly ever need to use them mid-combat. Just put up whatever you need or want up there.
I use a few of my extra Hotbars for easy click targeting single target party mitigations like Oblation and Intervention and Rescue when playing Healer.
The HUD is essential in communicating information to the player like cooldowns on attacks and abilities, health, and incoming and outgoing attacks. There is an immense amount of information to understand and process and any time spent looking around for information can be better spent on refocusing on the fight and responding to fight mechanics.
There are an infinite number of ways to display, hide, and arrange your HUD elements, so here's some guidelines when deciding what to do with a HUD element.
Where do you want to group your most important information together so it's all in one place?
How important is that piece of information? Does it need to be large or small?
How tight is the general area where your information is? Is it easy or hard to distinguish that element from neighboring elements?
Does the detailed or simple Job Gauge help you sort through and interpret that data more?
Does the element need to be there in combat?
Below are two videos with other specific HUD tips and considerations and designs.