I want to make my raspberry pi 3 B+ as a wifi router with a external wifi adapter for better speed, will attach a portable HDD/Pendrive to download file remotely by my phone from any place, And Stream downloaded media files in my phone when I am connected into pi's wifi network. Are these all possible to work simultaneously? If possible how? Also suggest me a wifi adapter which is compatible with Pi 3B+ .
Adding an external adapter is unlikely to increase speed - it is more likely to reduce speed.
All external I/O on the Pi is limited to the maximum throughput of a USB2 hub. For this reason the Pi makes a poor router.
The internal WiFi adapter uses the sdio interface, so is independent.
There should be no problem implementing multiple tasks on the Pi.
See https://www.pidramble.com/wiki/benchmarks/networking and https://www.jeffgeerling.com/blog/2018/raspberry-pi-3-b-review-and-performance-comparison for some networking tests on the Pi.
As far as I understand you will make your RasPi an access point and connect it to the internet. You don't tell us how do you want to connect it to the internet. You can use an USB dongle working as 4G modem or if you have already an internet router running you can uplink to it by wifi or wired. Downloading and streaming files to/from an attached USB HDD/Pendrive should also be possible. But as @Milliways already stated in his answer it is better to use the built-in wifi device for performance reasons. If you use an USB/wifi dongle it has to share the throughput with the Pendrive.
No matter what uplink do you use, if you have a supported device you will get interfaces eth0, eth1 and wlan0 and maybe usb0 out of the box. You can use these interfaces to make an uplink from the access point to the internet. If you have a wired connection (eth0) or a 4G dongle (eth1 or usb0) you can look at Setting up a Raspberry Pi as an access point - the easy way. If you want to uplink the access point by wifi you may have look at Access point as WiFi repeater, optional with bridge.