I made a PCB board with many components in eeschema editor. Then I had to assign footprints in Tools --> Assign footprints. Here when I assigned footprints for each components, these following items doesn’t seem to have footprints here or I’m not able to find them: opamp AD8620, transistor Q_NPN_CBE and transistor Q_PNP_CBE. Can you guys tell me where can I find their footprints. I’m new to KiCad.
You would assign them one of the industry standard packages like TO92, SOT23, SOP8, etc. In most cases.
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Right, the footprint you choose is based on the packaging of the component you plan to use, not its function.
But if I go to the assign footprint section I cannot see any opamps or transistors there
It all depends on specifically which part you are using, and which of the offered package types you are using in your design. Here is a web page with some of the through hole transistor packages (I’m sure there are more, these are just the more common ones):
https://eesemi.com/to-types.htm
For that OpAmp, are you using the MSOP or the SOIC package?
I’m sorry, but there is no simple answer. You need to get the datasheets for the parts you are using to see which packages are used for your parts, and then check which packages you can (or want to, some packages are more expensive than others) buy.
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Package outlines are not specific to part function. For example for TO92 you will find transistors (BJT and FET), voltage regulators, temperature sensors, even once an entire radio IC. As SembazuruCDE says, you need to decide on what package you will be using, and that will determine what footprint to assign. Building a PCB to hold components is where a conceptual design is literally materialised.
With **Eeschema / Tools / Assign Footprints ** you get to use CvPcb to fill in the footprint fields. This program has some powerful filters built in that help you narrow down the components you seek in the library… or you easily filter to much out and can not find anything anymore.
It takes some time to learn to work with it. Read the manual.
The most important part of CvPcb are the filter buttons: 
If these On/Off switches are set wrong for your search you completely mess up any search you do.
For some time there are alternatives to assign footprints to schematic symbols.
For example: Eeschema / Tools / Edit Symbol Fields or simply hover over a schematic symbol and press “F” for Footprint. then press [Select] to get to the footprint browser, where you can double click on a footprint name to select it.
Be very careful with footprints. This is not an issue which is specific to KiCad but rather applies to all modern pcb design. Footprints can be very confusing, especially when comparing between different manufacturers. Sometimes package names which seem very similar are significantly different, and often manufacturers will have different names for compatible packages. Many semiconductors offer packaging options which differ significantly. If you are not locked into a given package (for example if you or your company already has the components) check the distributor to see which packages are most available. Try not to commit your design to a specific device and package which is not easily available. Use the manufacturer’s datasheet and check the package dimensions very carefully and make sure your KiCad footprint will fit it the way you want. If you do not take a lot of care with this before you order boards, you will wish you had done so when you get the boards and the parts do not fit. Finally, do not fear surface mount. Except for the fine pitch and BGA devices, working with SMT on a properly designed board often is easier than working with through hole, especially if you need to change parts after assembly.
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You might want to start with a beginners tutorial Tutorial: Introduction to PCB design with KiCad version 5.1 (Getting Started)
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