If you are not using a specialised module for Google Tag Manager (GTM), there are 2 spots you can add the specific GTM snippets. The Header HTML Scripts and in a content block that is across the website.
If you are only using GA4 (Google Analytics 4), then you will only need to paste this code in the Header HTML Scripts section of Design > Configurations.
Generally, the first snippet is a script you can add to the <head> section of the website. To access this, you must be a marketplace owner admin.
1. Navigate to Content > Design > Configuration
2. Look for the store view that you would like to add the code to. For the most part, it's generally the last row. If you have multiple rows, please ensure you are selecting the correct store view.
3. Here you can see the design configurations - click on the section labelled HTML Head.
4. This will then expand it, and you can then look at for the field Scripts and Style Sheets. This is the section where you can add additional scripts from any third party, or custom scripts developed for your marketplace. This is where you can place the <head> Google Tag Snippet you are provided from Google.
5. Once pasted, simply choose Save Configuration (or Save and Continue if you wish to remain on this page).
Body Tag
For the body tag, you should be able to just add this into the website through the use of content blocks. You can use any of the header blocks, or you can consult with your support team on which block is best to use. It's an invisible <iframe> in general, so it shouldn't affect your design.