Generally, external drives are not designed to be used as the main boot drive, if you are planning on using it that way, But if you are using it for storage internally the same way you would use it as an external drive it will be fine.
You can make it by ripping out the disk and SATA cable from external HDD and connect it to your computer but before doing this make sure you have taken the backups.
The main thing you need to look at first Do you have a port to plug it into on the motherboard? & second thing Do you have enough power connectors on the power supply and does the PC have enough power to run the system with an extra device attached?
Unless it’s a 2.5mm drive it’s not going to fit the second thing is probably only room for 1 drive so unless the drive is way bigger than what’s in it I’d leave it alone.
If there is data on the drives you would need to back it up and format it and install the os on the new drive. May be you need to reinstall the operating system.
Just be aware of static electricity while handling the drive or working on the laptop.