For the best picture and sound quality, post a link to the video in the chat window of Google Meet and ask your students to click the link, view the video, and return to the Google Meet afterwards.
However, if your students watch a video and you want to show clips from that video during lecture, you can take advantage of a feature Google recently added to Meet that allows you to share a video with your students. You can use this feature for longer videos, but the quality of the video may be degraded by the students' internet connections.
You'll need to be running Chrome in order to take advantage of this:- Open your video (without playing it yet) in a separate tab. You might want to do this before the meeting begins, just to have it ready.

- When you're ready to play the video, mute your microphone. The audio from the video will come directly from the tab, and any sound from your mic will make the audio from the video cut out. (Obviously, feel free to unmute if you need to make commentary on the video and you don't mind preempting its audio.)
- In Google Meet, click Present now and choose A Chrome tab from the menu.

- Choose the tab with the video, make sure Share audio is checked, and click the blue Share button.

- Switch to the tab with the video. You'll see a message saying that you're sharing it in Google Meet.

- Play your video. If you click the "full screen" option, it will take up the entire tab and make the video easier for your students to see.

Google Meet does not include alternate backgrounds in the native application, but you can achieve this functionality by installing a separate program on your computer.
For PC users, one popular program is Chromacam: https://www.chromacam.me/#pricing The free version includes a small watermark that is relatively unobtrusive. You can also purchase an upgraded version for $30. Once you download and install the software, you can use it to select from a variety of backgrounds and filters. From within Meet, you will then go into settings to select Chromacam as your camera and then any background or effect that you set in Chromacam will appear in your webcam feed.
For both PC and Mac users, an alternative option is Snap Camera. https://snapcamera.snapchat.com/ Snap Camera is more of a novelty tool (it comes from the same company as SnapChat), but it does have a wide variety of backgrounds and some can be used for professional purposes. Users should be aware that not all backgrounds and filters are appropriate for professional or educational settings, so it is important to vet any of the filters before using them during class. Similar to Chromacam, you will download and install the software and will then have the option to choose Snap Camera as your webcam in Google Meet.