If you had access to the entire website, server logs, Search Console, analytics and development team, what would your first Technical SEO investigation be?

I would first establish the site's search architecture.

Step 1 — Understand the business

Identify:

• primary revenue pages;
• lead-generation pages;
• informational content;
• geographic markets;
• important user journeys.

Step 2 — Build the URL inventory

Compare:

CMS/database URLs

against:

sitemap URLs

against:

crawler-discovered URLs

against:

server-log URLs

against:

Search Console URLs

Step 3 — Understand crawl behavior

Determine what Googlebot actually spends time crawling.

Step 4 — Understand indexation

Determine which important URLs are:

• indexed;
• discovered but not indexed;
• crawled but not indexed;
• canonicalized elsewhere;
• blocked;
• noindexed.

Step 5 — Audit rendering

Select representative templates and compare:

HTTP response → rendered DOM → Google-rendered output

Step 6 — Audit architecture

Map:

internal links → canonical URLs → sitemap URLs

Step 7 — Identify the largest constraint

The first major finding might be:

• inefficient URL generation;
• poor discoverability;
• rendering failure;
• canonicalization;
• migration errors;
• server capacity;
• indexation quality.

Only then would I build the technical roadmap.

The central principle is:

Find the system-level constraint before fixing individual symptoms.

Google's documentation repeatedly emphasizes understanding the crawl/index/serving pipeline, managing URL inventory and validating how Google processes pages.