Let me put a few things out of the way first.... pardon the rather long read.
There are several ways to make your lights, smart:
1. Replace standard bulbs with smart bulbs - This is the fastest and easiest option. Most smart bulbs can be turned on/off, dimmed, or change color and/or scenes (dancing lights, etc.), via the app or a digital assistant (Alexa or GH). The downside is cost. The cheapest quality smart bulbs are from Yeelight. "Cheap" is of course relative and they still cost around P1k a pop from online shops like Shopee. If money is no object, there's also the Philips HUE system.... can't go wrong with those. Another disadvantage is the bulb needs continuous power. Therefore, you will need to keep the physical light switch turned on for every smart bulb it's connected to. If somehow, a family member or visitor turned off the physical switch, the smart bulb will lose power and you will no longer be able to operate it remotely.
2. Keep your standard bulbs/light fixtures and use a smart switch - There are two sub-categories of smart switches:
a. WiFi Smart Switch - These switches can be controlled manually, through the app, or digital assistants. As these switches are active switches, you will be able to see the realtime status of the switch (whether it's on or off) in the app. You can even ask GH the status of the switch and it will tell you if it's on or off. The downside is, the switch will very likely require a Neutral wire to keep itself powered up. The Neutral wire is an electrician-speak for having 220V supply available on every smart switch location in your home. To put things into perspective, houses made in the last century does not have a Neutral wire available on light switches as they're not needed. So it stands to reason that some wiring modifications are required in your house electrical to use this type of smart switch.
b. Neutral-less RF Smart Switch - Similar capabilities as above but does not require a Neutral wire. It works off the residual electricity available on the light circuit for its own power. These are passive switches and are usually controlled via 315 or 433MHz RF (not WiFi) and requires a hub. The downside is, as these are passive, there's no way to remotely determine if the switch is on or off through the app or GH.
These smart switches are on/off only. They are not capable of dimming the light. But just to be complete, there are expensive ones available on Amazon or eBay that can dim. They also work via RF, but uses a different protocol such as Z-Wave or Zigbee. These too, requires a hub.
Now, on to your questions:
Any of the approach I mentioned above will work. Further, you can also set timers or schedules. I believe a random setting is available that will randomly turn on/off your lights while no one's around to give your home the effect that somebody's home. You can also use a 3rd party cloud service such as
"If This Then That" (IFTTT) to create conditions and triggers to do things like turn on your lamp shades whenever your GH or Alexa alarm goes off or turn your colored bulbs purple when it's about to rain... things like that.
There are two ways you can do this; Create a Scene or a Routine (in GH or Alexa).
Same response as your #1 question. This can be done, and more, using the respective apps.
As always, I can only speak based on my own preference and experience. That said, I can never recommend enough Broadlink products for replacing the standard light switches and Yeelight Color v2 for smart bulbs.
My plan is to use Broadlink TC2 light switches for the MAIN lights in ALL rooms (the lights you typically turn on when entering the room). I then use smart colored bulbs for accent lighting, lamp shades or where a colored bulb is desirable (ie. corner lighting). Right now, I mostly only have the cheap Chinese knock-off smart bulbs but I might replace them with Yeelights in the future for reasons of safety.
To start you off, you only need the following:
1. GH or Echo (Alexa)
2. Smart bulbs of your choice (as many as required)
3. Broadlink TC2 switches (as many as required) and one (1) Broadlink RM Pro hub to control them
4. A mobile device to install, run, and configure the necessary apps
5. WiFi and internet
That's it.