Sometimes you need to pin an app that is part of the core Chrome OS to the shelf. Common requests include the Files app, Camera, and parts of the productivity suite. This can be achieved by selecting your OU, going to "Apps & Extensions", then "Managed guest sessions". In the bottom right, you will see a + icon. Hover over it and select "Add Chrome app or extension by id". Enter you extension ID then change the dropdown from "From the Chrome Web Store" to "From a custom URL". The extension ID will look something like "hhaomjibdihmijegdhdafkllkbggdgoj", and the URL for native apps will always be "http://clients2.google.com/service/update2/crx" (may need to change it to https).
Once the app is added to your OU's list, you can set the app to force install, force install + pin to shelf, or not install.
For a non-native app you can add the app via the webstore if available or directly by the extension ID.
Check out some frequently used apps and extensions in libraries here.
Some common extension IDs for native and Web Store apps:
Adobe Acrobat - efaidnbmnnnibpcajpcglclefindmkaj
Adblock Plus - cfhdojbkjhnklbpkdaibdccddilifddb
Application Launcher For Drive (by Google) - lmjegmlicamnimmfhcmpkclmigmmcbeh
Cisco Webex Extension - jlhmfgmfgeifomenelglieieghnjghma
DuckDuckGo Privacy Essentials - bkdgflcldnnnapblkhphbgpggdiikppg
Office - ndjpnladcallmjemlbaebfadecfhkepb
Google Keep Chrome Extension - lpcaedmchfhocbbapmcbpinfpgnhiddi
Zoom Scheduler - kgjfgplpablkjnlkjmjdecgdpfankdle
Grammarly for Chrome - kbfnbcaeplbcioakkpcpgfkobkghlhen
Zotero - ekhagklcjbdpajgpjgmbionohlpdbjgc
Files App - hhaomjibdihmijegdhdafkllkbggdgoj
Google Mail - pjkljhegncpnkpknbcohdijeoejaedia
Google Drive - apdfllckaahabafndbhieahigkjlhalf
Google Docs - aohghmighlieiainnegkcijnfilokake
Google Docs Offline - ghbmnnjooekpmoecnnnilnnbdlolhkhi
Google Sheets - felcaaldnbdncclmgdcncolpebgiejap
Google Slides - aapocclcgogkmnckokdopfmhonfmgoek
YouTube - blpcfgokakmgnkcojhhkbfbldkacnbeo
Chrome PDF Viewer - mhjfbmdgcfjbbpaeojofohoefgiehjai
Chrome Cast - pkedcjkdefgpdelpbcmbmeomcjbeemfm
You can find the extension IDs of apps installed on a Chromebook by going to: chrome://system/#extensions