Archive catalogues may be in online or hard copy format. It is common for archives to have a combination of the two. Not everything held by the archive may be catalogued and some items may be catalogued only on paper, usually in the format of old typescripts.
Archives are organised and catalogued in a hierarchical pattern, which can help to show the record in context and identify other relevant records.
Information you might find in a catalogue record includes the provenance of the item, dates of creation, a physical description, some relevant administrative or biographical history and any conditions of access. Some catalogue records will link to digital versions of the item, but others will simply contain information about the item and you will need to visit the archive to see it or make use of the archive's copying service if they offer one.
Be methodical and record your searches as you go
Search for variant/archaic spellings
Look at the help or guidance pages of the catalogue to see local search tips, e.g.
how to search for phrases
how to combine terms
how to use wildcards
how to enter dates